We measured SO2, NO2 and O-3 concentrations during the summer and winter in Kocaeli, Turkey. The sampling was carried out indoors and outdoors at homes, schools and offices. Personal samplers were also used to determine personal exposures to these pollutants. High NO2 and SO2 concentrations were observed in outdoor samples collected close to locations characterized by heavy urban traffic. Concentrations of O-3, on the other hand, were higher in rural areas around the city due to ozone distillation. For both sampling periods, the concentrations of outdoor SO2 and O-3 were higher than for indoor and personal samples; however, the NO2 concentrations were higher in indoor and personal samples, indicating that outdoor sources significantly contribute to indoor SO2 and O-3 levels and that indoor NO2 concentrations are primarily modulated by sources within buildings. Seasonal variations in pollutant concentrations showed statistically significant differences. Indoor and outdoor concentrations of NO2 and SO2 measured in the winter were higher than the levels measured in the summer; O-3 concentrations, on the other hand, exhibited the opposite trend. Active-to-passive concentration ratios for NO2, SO2 and O-3 were 0.99, 1.08 and 1.16, respectively; the corresponding outdoor ratios were 0.95, 0.99 and 1.00.