The seasonal and spatial characteristics of PM2.5 and its chemical composition in the Mediterranean Basin have been studied over a 1-year period (2011-2012) in five European Mediterranean cities: Barcelona (BCN), Marseille (MRS), Genoa (GEN), Venice (VEN), and Thessaloniki (THE). During the year under study, PMuj annual mean concentration ranged from 23 to 46 mu g m(-3), while the respective PM2.5 ranged from 14 to 37 mu g m(-3), with the highest concentrations observed in THE and VEN. Both cities presented an elevated number of exceedances of the PM10 daily limit value, as 32% and 20% of the days exceeded 50 mu g m(-3), respectively. Similarly, exceedances of the WHO guidelines for daily PM2.5 concentrations (25 mu g m(-3)) were also more frequent in THE with 78% of the days during the period, followed by VEN with 39%. The lowest PM levels were measured in GEN. PM2.5 exhibited significant seasonal variability, with much higher winter concentrations for VEN and MRS, in fall for THE and in spring for BCN. PM2.5 chemical composition was markedly different even for similar PM2.5 levels. On annual average, PM2.5 was dominated by OM except in THE. OM contribution was higher in Marseille (42%), while mineral matter was the most abundant constituent in THE (32%). Moreover, PM2.5 relative mean composition during pollution episodes (PM2.5 > 25 mu g m(-3)) as well as the origins of the exceedances were also investigated. Results outline mainly the effect of NO3- being the most important driver and highlight the non-negligible impact of atmospheric mixing and aging processes during pollution episodes. (C) 2014 Elsevier B.V. All rights reserved.