Antifoulant Irgarol 1051 (2-methythiol-4-tert-butylamino-6-cyclopropylamino-s-triazine) can be photodegraded into M1 (2-methylthio-4-tert-butylamino-6-amino-s-triazine) and M2 (3-4-tert-butylamino-6-methylthiol-s-triazin-2-ylamino]propion-aldehyde). M3 (2-methylthio-4,6-bis-tert-butylamino-s-triazine) was also detected as a side-product in Irgarol. This study aimed to investigate the combined toxicity of a mixture of these s-triazine compounds to eight marine organisms. A degraded mixture of Irgarol in artificial seawater was obtained by photolysis over 42 d and its composition was quantified by HPLC-UV analyses. Based on short-term toxicity tests on eight selected marine species, the mixture posed significant phytotoxic effects to the cyanobacteria (Chroococcus minor and Synechococcus sp.), the diatoms (Skeletonema costatum and Thalassiosira pseudonana), the macroalgae (Ulva lactuca and Caulerpa peltata) and the dinoflagellate (Prorocentrum dentatum), though the mixture was less toxic to the copepod Tigriopus japonicus. Both Independent Action and Concentration Addition models can generate reasonably satisfactory predictions on the overall mixture toxicity to the two diatoms, implying that the four compounds likely share a similar mode of action and resemble an additive effect in the mixture.