How Much MTBE Is Added to Gasoline?

Where MTBE is used, gasoline typically contains about 10–15% MTBE by volume: roughly 11% vol to meet earlier U.S. reformulated gasoline oxygen requirements (about 2.0 wt% oxygen) and up to around 15% vol for winter oxygenated programs (about 2.7 wt% oxygen). Today in the United States, MTBE use in retail gasoline is effectively zero due to state bans and liability concerns; ethanol has replaced it as the dominant oxygenate.

What MTBE Is—and Why It Was Added

Methyl tert-butyl ether (MTBE) is an oxygenate and high-octane blending component once widely used to help gasoline burn cleaner and to reduce carbon monoxide and ozone-forming emissions. It rose to prominence in the 1990s under Clean Air Act programs that required minimum oxygen content in certain fuels. However, MTBE is highly water-soluble and persistent, and leaks from storage or spills led to groundwater contamination issues, prompting a widespread phase-out in the U.S. in the mid-2000s.

Typical Blending Levels and Oxygen Equivalence

The amount of MTBE added depends on the oxygen content target and local fuel specifications. The figures below summarize common blend levels historically used to meet regulatory programs and optional performance goals.

• Reformulated Gasoline (RFG) areas: about 11% MTBE by volume (≈2.0% oxygen by weight in the finished gasoline).
• Winter Oxygenated Gasoline (CO reduction): about 15% MTBE by volume (≈2.7% oxygen by weight).
• Conventional gasoline (optional octane boost): 0–8% by volume, when permitted by local rules and economics.

These ranges covered most operational needs: roughly 11% vol MTBE met the routine RFG oxygen minimum, while ~15% vol met stricter winter oxygen requirements. Lower discretionary doses were sometimes used purely for octane enhancement when no oxygen mandate applied.

Current Status by Region (2025)

MTBE’s prevalence today is driven by environmental policy and market alternatives like ethanol or ETBE. Here is where things stand in broad terms:

• United States: Virtually no MTBE in retail gasoline. Many states banned it (2004–2006), and the 2005 Energy Policy Act removed the federal oxygen mandate for RFG. Ethanol (typically up to 10% vol, and higher in some blends) is the standard oxygenate.
• Canada: MTBE use is minimal to none in most provinces; ethanol dominates as the oxygenate/renewable component.
• European Union: MTBE remains permitted and is used to varying degrees, though ETBE (often sourced from bioethanol) and ethanol are common alternatives. Usage levels vary by country, refinery configuration, and renewable mandates.
• Middle East and parts of Asia: MTBE can still be used as an octane booster/oxygenate where regulations allow; adoption depends on local environmental policies and supply economics.
• Latin America and other regions: Mixed practices; some countries allow MTBE, others rely primarily on ethanol or ETBE depending on domestic supply and air-quality goals.

In short, the U.S. market has moved almost entirely to ethanol, while MTBE persists in some international markets where groundwater concerns are addressed via regulation and infrastructure or where alternative oxygenates are less economical.

What Determines the MTBE Dose?

Even where MTBE is allowed, the exact percentage depends on multiple engineering and policy factors. The items below outline the main drivers.

• Regulatory oxygen targets: Programs historically required 2.0–2.7 wt% oxygen; MTBE content was set accordingly (≈11–15% vol).
• Vapor pressure limits (RVP): Blenders balance MTBE against other components to meet seasonal volatility caps.
• Octane needs: MTBE has high blending octane; more may be used to reach target RON/MON if aromatics must be reduced.
• Aromatics and benzene caps: MTBE can help meet toxics limits by displacing higher-aromatic streams.
• Compatibility with ethanol: Co‑blending strategies consider material compatibility, water tolerance, and distribution systems.
• Supply economics: Availability and price of MTBE, ethanol, and refinery blendstocks influence the chosen oxygenate.
• Local environmental restrictions: Groundwater protection policies may limit or prohibit MTBE regardless of refinery needs.

Refiners balance emissions compliance, performance, and cost. Where MTBE is permitted, its final share reflects both the oxygen target and broader fuel-spec tradeoffs.

Numeric Quick Reference

MTBE contains about 18.2% oxygen by weight. Because MTBE’s density is close to that of gasoline, the following approximations are widely used:

• 10% MTBE by volume ≈ 1.8% oxygen by weight in finished gasoline.
• 11% MTBE by volume ≈ 2.0% oxygen by weight (typical RFG minimum).
• 15% MTBE by volume ≈ 2.7% oxygen by weight (typical winter oxygenated fuel target).
• Octane effect: About 10% MTBE by volume commonly raises blend RON by roughly 2–3 numbers (actual gain varies with base gasoline).

These rules of thumb help translate between regulatory oxygen targets and practical blend percentages while giving a sense of the octane benefit.

Health and Environmental Considerations

MTBE’s principal drawback is its mobility and persistence in water. Even small leaks can impart taste and odor at very low concentrations, prompting costly remediation. This risk—rather than in-vehicle performance—drove the policy shift away from MTBE in the U.S. toward ethanol and other components.

Summary

Where used, MTBE is typically blended into gasoline at about 10–15% by volume: around 11% for reformulated gasoline needs and up to 15% for stricter winter oxygen requirements. In the United States today, MTBE use in retail gasoline is essentially nil, with ethanol serving as the primary oxygenate. Internationally, MTBE remains allowed and used in some markets, with the exact dosage governed by oxygen targets, fuel specs, economics, and environmental regulations.