Critical micelle concentrations (CMCs) of the penicillins cloxacillin and dicloxacillin in water were determined by conductivity measurements over the temperature range 288.15 to 313.15 K. Both penicillins showed minimum CMCs at temperatures close to 298.15 K. Thermodynamic parameters of aggregate formation were derived from the variation of the CMC with temperature using a modified form of the mass action model applicable to systems of low aggregation number. Values for the enthalpy of aggregate formation, Delta H-m(0), calculated by this method showed that the aggregation of both cloxacillin and dicloxacillin became increasingly exothermic with increase in temperature. The predicted Delta H-m(0) at 298.15 K was in good agreement with the value determined experimentally by calorimetry for each drug.