Volatile organic compounds (VOC) play a significative role on the process of formation of photochemical smog. Photooxidants are formed in the atmosphere when VOC react with nitrogen oxides (NOx), in the presence of sunlight and OH radicals. In urban areas, vehicle fuels constitute an important source of VOC and NOx emissions to the atmosphere, either by burning or evaporative losses. This work is concerned with a comparative study of two types of light-duty vehicular fuels presently used in Brazil - namely gasohol (a mixture 22-24% of anhydrous ethanol in gasoline) and hydrated ethanol (95% v/v) - as potential precursors for ozone formation in the atmosphere of urban cities. The experiments were conducted during the spring and summer of 2001/2002. In each one, two Teflon chambers were filled, respectively, with each fuel, at high initial [VOC] to [NO] ratios and exposed to the sunlight. Ozone formation, as well as NOx concentration profiles were measured directly using ozone and NO/NO2/NOx continuous analyzers. The results showed that, for the same initial volumes of the two fuels into the bags, the ozone peak concentrations are in average 28% higher for alcohol than for gasohol. In addition, the ozone formation process showed a tendency to start earlier for gasohol but, once started, the formation rate was two times faster for alcohol than for gasohol. These observations indicate a need for more detailed studies, regarding the role of unburned fuel emissions and evaporative losses of alcohol and gasohol on the formation process of atmospheric photochemical oxidants in urban sites, with special attention to kinetics aspects. This conclusion is particularly important in the case of Brazil, considering the substantial contribution of that emissions on the total hydrocarbons (HC) and VOC inventories from mobile sources and a possible resume of the Brazilian Biofuels Program, which would increase the atmospheric concentrations of ethanol.