Alkyl bromides afford the corresponding sulfide derivatives upon treatment with aqueous sodium sulfide under phase-transfer catalysts (PTC) and ultrasound irradiation conditions. The use of phase-transfer catalysis and sonochemistry has been compared and demonstrated in these nucleophilic substitution reactions. The kinetics of the reaction depends on the effect of amount of catalyst, agitation speed, quaternary ammonium salts, amount of sodium sulfide, amount of sodium hydroxide, organic solvents, temperature and the frequency of the ultrasound waves on the conversion of alkyl bromides were investigated in detail. A kinetic model was built from which a pseudo-first order rate law is sufficient to describe the behavior of the reaction. The observed peculiar phenomena were also explained satisfactorily. (c) 2005 Elsevier B.V. All rights reserved.