The levels of carbonyl compounds were determined at the roadside urban station at the Hong Kong Polytechnic University (HKPU) campus during January 2002 to February 2002. Nine carbonyl compounds were quantified in this study. Temperature and solar radiation were found to affect the photochemical reactions of the carbonyls. Formaldehyde/acetaldehyde ratio ranged from 1.27 to 1.35. Strong correlations between formaldehyde and acetaldehyde were found, in the time period 1800-2100, which indicated that they were originated from the same sources during this time period. Roadside carbonyl samples were also collected at four other roadside environments during 2001. Kwai Chung (KC) station showed the highest average formaldehyde and acetaldehyde concentrations due to its highest traffic flow, especially for diesel vehicles. High concentration of toluene emitted from gasoline-fueled vehicles was believed to be the cause of high benzaldehyde level at the Central (CT) station through the photochemical oxidation of toluene. ne average concentrations of formaldehyde and acetaldehyde in Hong Kong are well within the ranges reported in roadside environments of other urban cities. However, Mexico City in Mexico and Cairo in Egypt had much higher concentration levels of formaldehyde and acetaldehyde than in Hong Kong roadside environment due to the incomplete combustion of different fuel compositions. (c) 2005 Elsevier B.V. All rights reserved.