Results from simultaneous measurements of aerosol chemical composition and optical thickness carried out on the Baltic Sea in July 1997 and March 1998, as a part of the Baltic Sea System Study (BASYS) Atmospheric Load project, are presented. The aerosol optical thickness data within the spectral range of 412-865 nm are analysed separately for summer and winter. Mie theory was used to calculate the extinction properties of black carbon, organic carbon. ammonium and sea-salt, taking into account, in the case of the last two components, the measured mass size distribution. Comparison of chemical and optical measurements, leads to the conclusion that sea-salt dominates the aerosol optical thickness values for alpha less than or equal to 0.26. For 0.26 < x less than or equal to 0.35 both sea-salt and mineral dust control the aerosol optical thickness. For 0.41 < alpha less than or equal to 0.56 the contribution from both mineral dust and organic carbon is significant. For 0.56 < alpha less than or equal to 0.97 both organic carbon and ammonium salts control the values of the aerosol optical thickness, For alpha greater than or equal to 1.47 the aerosol optical thickness values are dominated by black carbon.