This paper uses a complicated set of phase-ins and phase-outs of oxygenated motor fuel in the Northeast to determine whether E-10 ethanol-enhanced fuel contributes to acetaldehyde air pollution over the pre-ethanol methyl tertiary-buthyl ether (MTBE) fuel. Oil companies phased out MTBE because of groundwater pollution concerns, and now E-10 is the standard fuel in EPA reformulated gas areas. Using a difference-in-difference setup, I find a small level increase but a large percentage increase in acetaldehyde pollution with E-10. I also compute a cost of the pollution in the single-digit millions of dollars. The findings concur with many scientific papers estimating that the impact of E-10 fuel on acetaldehyde pollution is small but positive.