Aerosol acidity (pH) affects aerosol composition and properties, and therefore climate, human health and ecosystems. Fine aerosol acidity and its seasonal variation at 6 sites (Finokalia, Patras, Thissio, Ioannina, Thessaloniki, and Xanthi) in Greece were investigated during 2019-2020. The thermodynamic model ISORROPIA-lite was used to calculate aerosol water and acidity based on measurements of the chemical composition of PM2.5 and available gas-phase concentrations of HNO3, NH3, and HCl. During winter the fine aerosols were acidic to moderately acidic throughout Greece with an overall mean aerosol pH of 3.57±0.44 in urban areas and 3.05±0.50 in remote locations. The highest aerosol pH (4.08±0.42) in January 2020 was found in Ioannina due to, among others, high K+ levels from biomass burning emissions. Aerosols in Xanthi were the most acidic due to high sulfate levels. Similar seasonal profiles of aerosol pH were observed at all sites studied with different factors contributing to this seasonality. During the summer PM2.5 at Thissio, Ioannina and Finokalia was acidic with a mean aerosol pH across all three sites of 1.76±0.40. During this season, sulfates were the driver of the higher acidity conditions at Thissio and Finokalia, with other factors such as the semivolatiles and temperature contributing to a lesser extent. At Ioannina, temperature along with the total ammonia and nitrate were the main contributors to the seasonal difference of the aerosol pH, while some of the nonvolatile species also contributed. In most cases, the importance of organics for aerosol pH was small.