Chloride ion is used as an efficient scavenger of hydroxyl radicals to measure the yield of HO• produced in the photolysis of acidic aqueous solutions of hydrogen peroxide. The scavenging efficiency is shown to depend on the chloride ion concentration and solution acidity, as expected from the mechanism. The results show that the quantum yield of hydroxyl radicals produced from hydrogen peroxide photodissociation in the aqueous phase is close to unity at both 248 and 308 nm:  ΦHO•(248 nm) = 1.0 ± 0.1 and ΦHO•(308 nm) = 0.8 ± 0.2, where the uncertainties (±σ) were obtained from propagation of errors. [Because of geminate recombination in the solvent cage, quantum yields in solution are often smaller than those in the gas phase, where the HO• quantum yields are equal to 2.] The present results for the aqueous phase are generally consistent with other determinations.