A combined experimental and theoretical study of the photochemistry of CHBr3 in pure water and in acetonitrile/water mixed solvents is reported that elucidates the reactions and mechanisms responsible for the photochemical conversion of the halogen atoms in CHBr3 into three bromide ions in water solution. Ultraviolet excitation at 240 nm of CHBr3 (9 x 10(-5) M) in water resulted in almost complete conversion into 3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-resolved resonance Raman (ps-TR3) experiments and ab initio calculations indicate that the water-catalyzed O-H insertion/Hbr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the production of the final products observed following ultraviolet excitation of CHBr3 in water. These results have important implications for the phase-dependent behavior of polyhalomethane photochemistry and chemistry in water-solvated environments as compared to gas-phase reactions. The dissociation reaction of HBr into H+ and Br- ions is the driving force for several O-H insertion and HBr elimination reactions and allows O-H and C-H bonds to be cleaved more easily than in the absence of water molecules. This water-catalysis by solvation of a leaving group and its dissociation into ions (e.g., H+ and Br- in the examples investigated here) may occur for a wide range of chemical reactions taking place in water-solvated environments.