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, logK OW , and organic matter–water partition coefficients, logK OC , of these s-triazines were measured by reversed-phase HPLC and a triphasic SPME equilibrium model, respectively. The average logK 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 logK 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.