An analytical method based on graphite-furnace atomic-absorption spectrophotometry employing a suitable signal-enhancing medium for determination of inorganic tin and two of its trisubstituted organic derivatives in sea-water has been established. This method allows determination of triphenyltin and tributyltin compounds down to 2 x 10(-12) and 2.8 x 10(-12)M respectively by means of enrichment by collection on graphitized carbon black (enrichment factor up to 8 x 10(4)) and a separation on a small silica-gel column. Inorganic tin, which is not adsorbed on the graphitized carbon black, is isolated from the matrix by liquid-liquid extraction of its pyrrolidinedithiocarbamate complex into dichloromethane. The method gives good recovery (95%) and precision ( less, similar5%) at the ng/l. level.