Recebido em 18/1/07; aceito em 27/4/07; publicado na web em 25/10/07 SIZE DISTRIBUTION OF ATMOSPHERIC PARTICULATE IONIC SPECIES AT A COASTAL SITE IN PORTUGAL. A Berner impactor was used to collect size-differentiated aerosol samples from March to August 2003 in the city of Aveiro, on the Portuguese west coast. The samples were analysed for the main water-soluble ion species. The average concentration of sulphate, nitrate, chloride and ammonium was 6.38, 3.09, 1.67 and 1.27 μg m -3 , respectively. The results show that SO 4 2-and NH 4 + were consistently present in the fine fraction < 1 μm, which represents, on average, 72 and 89% of their total atmospheric concentrations, respectively. The NO 3 -particles were concentrated in the coarse size. Chloride presented the characteristic coarse mode for marine aerosols. During some spring/summer events, an ammonium surplus was observed (NH 4 + /SO 4 2-molar ratios > 2), possibly due to greater availability of ammonia coming from agricultural activities or from the neighbouring chemical industrial complex. During the remaining periods, the aerosol was found to be somewhat acidic and predominantly in the form of ammonium bisulphate (NH 4 + /SO 4 2-molar ratios = 0.5-1.25). Samples collected under a major or exclusive influence of maritime air masses were essentially constituted by coarse particles with enrichment in sea salt, while for air masses of continental origin the contribution of water-soluble ionic species in the fine mode was more pronounced.