Water temperature and pH affect the toxicity of most chemicals to aquatic organisms. The objectives of the study were to determine: (1) individual and interactive effects of water temperature (7, 12, 17C), pH (6.5, 7.5, 8.5, 9.5), and time on the acute toxicity of terbufos, trichlorfon, 4-nitrophenol, and 2,4-dinitrophenol to rainbow trout (Oncorhynchus mykiss) and the amphipod Gammarus pseudolimnaeus; (2) individual and interactive effects of water temperature and pH on chemical bioconcentration in acute tests with rainbow trout and Gammarus exposed to terbufos, 4-nitrophenol, and 2,4-dinitrophenol. The toxicity of all four chemicals was significantly affected by pH in all tests except for Gammarus exposed to terbufos. Temperature significantly affected toxicity of all chemicals to both species. Toxicity increased with temperature in all tests except for rainbow trout exposed to nitrophenol. Significant interactive effects between toxicity modifying factors were observed in nearly all tests. Temperature and pH effects on chemical toxicity need to be considered in chemical hazard assessment to ensure adequate protection of aquatic organisms.