The activity concentrations of dissolved 137 Cs have been determined in the water column and 137 Cs and 134 Cs in the sediments and the sediment porewaters of the southern Baltic Sea. The mean activity concentration of dissolved 137 Cs in the Gdansk Deep declined from 109Bq m −3 in June 1986 to 61Bq m −3 in 1999. In sediments, the activity concentrations of 137 Cs (33-231Bq kg −1 ) were highest in muds and the activity concentrations of 134 Cs were about 6% of the total Cs activity. The Chernobyl contribution to 137 Cs activity was between 43% and 77%. The porewater activity concentrations of 137 Cs in muddy sediments were in the range 71 to 3900Bq m −3 and were higher than those in the overlying seawater. The diffusive flux of dissolved 137 Cs from the muddy sediments was estimated in the range 5 to 480Bq m −2 year −1 . The flux of 137 Cs from sediment porewaters of the southern Baltic Sea was about 45% of the total, including fluxes of 137 Cs from wet and dry atmospheric deposition and the fluvial inputs. The results were used to elucidate the rate of recovery of the sediments and the waters of the southern Baltic from Chernobyl-derived 137 Cs.