The MCMA 2003 field campaign was a comprehensive investigation of photochemical gas and aerosol air quality in the Mexico City megacity conducted during April, 2003. A preliminary study was also conducted during February, 2002. As part of this study, ambient VOC samples were collected from airshed boundary sites, central urban core sites and downwind urban receptor sites. Samples were also obtained during mobile vehicle chase operations designed to obtain data on direct vehicle emissions. In this paper, preliminary results from the VOC measurements are presented in terms of the distribution of VOC species and the magnitudes and diurnal patterns of VOC levels. The use of ratios of individual VOC species are used to characterize different sites, to investigate the relative reactivity of different species, and for comparison to similar analyses conducted in the United States. VOC ambient distributions are also compared to distributions in the available emission inventory for Mexico City.