Atmospheric aerosols, PM2.5 were simultaneously collected in the center of Athens and in a semi-urban area of the Athens basin, using the Harvard Impactor (HD) system, from March 1995 to March 1996. 224 24-hr samples were collected. Chemical analysis of the filter samples was performed using ion chromatography (Cl-, SO4(2-); NO3-, Na+, K+, NH4+, Mg2+, Ca2+). In addition aerosol addity was measured using a semi-micro electrode. No significant differences in chemical composition of particles were observed between the two sampling sites. At the city center the annual-average of non-sea-salt sulfate concentration was 85 nmoles m(-3). Also the annual mean values of chloride, ammonium and sodium concentrations were 48, 88 and 71 nmoles m(-3) respectively. Lower concentrations were observed for the rest of the ions. Aerosol acidity was higher at the city center, 18 nmoles m(-3), compared to that observed at the semi-urban site, 14 nmoles m(-3). Species concentrations were examined by season. The mean monthly concentrations of Cr, NO3-, Ca2+ and H+ were higher in the winter. In contrast those of Mg2+ Na+ and K+, were higher in the summer and the spring, respectively. The concentrations of SO4(-2)and NH4+ ions did not exhibit a seasonal pattern. Sulfate and ammonium ions were the predominant ionic species and their ionic ratio ranged between those of ammonium sulfate and letovicite.