Ozone is formed in the troposphere through a complex series of photochemical reactions involving oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Atmospheric scientists conduct field measurements of these and other trace gases to validate and improve air quality models, which can then be used to test various mitigation strategies. One such field campaign occurred during March 2006 in Mexico City, one of the world's largest cities. During the campaign, we determined concentrations of hydrogen peroxide, a photochemical termination product, aboard the U.S. Department of Energy research aircraft, and at a surface site northwest of the city. We observed surprisingly low peroxide mixing ratios near the source region, and decreasing abundance downwind of the city. However, relatively high values of peroxide were observed at takeoff and landing near Veracruz, a site with much higher humidity and lower concentrations of NOx. Our observations are consistent with the accepted mechanism of peroxide formation in a high-NOx, low humidity environment.