A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr4, in water and acetonitrile/water was performed. Ultraviolet photolysis of low concentrations of CBr4 in water mostly leads to the production of four HBr leaving groups and CO2. Picosecond time-resolved resonance Raman (Ps-TR3) experiments and ab initio calculations indicate that water-catalyzed O-H insertion/HBr elimination of the isomer of CBr4 and subsequent reactions of its products lead to the formation of these products. The UV photolyses of di-, tri-, and tetrahalomethanes at low concentrations in water-solvated environments are compared to one another. This comparison enables a general reaction scheme to be deduced that can account for the different products produced by UV photolysis of low concentrations of di-, tri-, and tetrahalomethanes in water. The fate of the (halo)formaldehyde intermediate in the chemical reaction mechanism is the key to determining how many strong acid leaving groups are produced and which carbon atom final product is likely formed by UV photolysis of a polyhalomethane at low concentrations in a water-solvated environment.