The data for formaldehyde and acetaldehyde levels in ambient air of the city of Rio de Janeiro, obtained in the period from 4 December 1998 to 17 January 2001 is presented. A total of 28 samples were collected at a downtown area, where emissions may be mainly attributed to the vehicular fleet. Values between 1.52 and 54.31ppb for formaldehyde and between 2.36 and 45.60ppb for acetaldehyde were obtained. The high acetaldehyde/formaldehyde ratios (0.76 to 1.61) are a consequence of the use of oxygenated fuels. Brazilian cities are unique in that the vehicles use hydrated ethanol (over 4 million of light duty vehicles), gasohol (a mixture with gasoline and 24% v/v of ethanol) and diesel fuels. The analysis of vehicle exhaust and model simulations of the air quality in August and December 1999, confirmed that the high levels of acetaldehyde could be attributed to direct emissions of the vehicular fleet and to the photochemical initiated oxidation of organic compounds.