Partitioning behavior of the antifouling booster biocide, Irgarol-1051 (2-methythio-4-tert-butylamino-6-cyclopropylamino-s-triazine), its production by-product, M3, and its environmental transformation products, M1 and M2, were studied. Octanol-water partition coefficients, log K(OW), and organic matter-water partition coefficients, log K(OC), of these s-triazines were measured by reversed-phase HPLC and a triphasic SPME equilibrium model, respectively. The average log K(OW) (+/-SD) of the four s-triazine species were: 4.39+/-0.07 (M3); 3.38+/-0.12 (Irgarol-1051); 2.92+/-0.12 (M2) and 2.54+/-0.11 (M1), while mean log K(OC) (+/-SD) of these species were: 2.47+/-0.03 (M3); 2.16+/-0.03 (Irgarol-1051); 1.97+/-0.03 (M2) and 1.79+/-0.04 (M1). These results were compared to reported physicochemical parameters of Irgarol-1051 in the literature. Partitioning behavior of these s-triazine species in the coastal environment revealed by their K(OW) and K(OC) were also discussed.