Ambient concentrations of sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3) were measured at 51 sampling points by passive sampling technique in Kocaeli, an important industrial city in Turkey. Samples were analyzed by UV-spectrophotometry for NO2 and O3 and by ion chromatography for SO2, respectively. Concentrations of SO2, NO2, and O3 were determined to investigate their spatial distribution and source characterization. The sampling campaigns revealed an average concentration of 8 mu g/m3 (max. 82 mu g/m3) for SO2, and 14 mu g/m3 (max. 40 mu g/m3) for NO2, in summer; while average winter concentrations were 25 mu g/m3 (max. 61 mu g/m3) for SO2, and 50 mu g/m3 (max. 100 mu g/m3) for NO2. The maximum ozone concentrations were determined to be 86 mu g/m3 in summer and 61 mu g/m3 in winter downwind of the source areas of the precursor pollutant emissions. The results showed that NO2 and SO2 concentrations in industrial and urban areas were two to four times higher compared with rural areas in the summer and winter. In the light of the information obtained from the spatial interpolation of the pollutant concentrations, a selection of appropriate locations for continuous monitoring was suggested according to the European Community (EU) directives.