Department of Chemistry, North Eastern Hill University, Shillong-793 022, India Manuscript received 4 November 1999, revised 12 April 2000, accepted 4 July 2000 The critical micellar concentrations (cmc) of sodium dodecyl sulfate (SDS) and cetylpyridinium chloride (CPC) in acetonitrile­water media of various compositions have been determined at different temperatures. The cmc of SDS slightly decreases with the increase in acetonitrile concentration and with the increase in temperature, whereas that of CPC increases with the increase in concentration of acetonitrile and temperature. An attempt has been made to explain this type of behaviour. Contrary to the general expectation, the micellization of SDS and CPC in mixed solvents seems to be enthalpy-controlled rather than entropy-controlled.