A simple thermodynamic analysis of ionic/nonionic mixed micelles in the media of moderately high ionic strengths is presented. A measure of the stability of mixed micelles is presented. According to the analysis, a contribution (\left(B_{1}\right)) is extracted in addition to the other interaction parameter (B_{2}) (an analog of the familiar parameter (\beta) in the regular solution approach to mixed micelles). The parameter (B_{1}) is related to the standard free energy change associated with the introduction of one ionic species into a nonionic micelle coupled with the release of one nonionic species from the micelle. This parameter (B_{1}) plays an essential role for a change of CMC values of nonionic micelles when an ionic species enters the micelle. When applied to dodecyldimethylamine oxide (DDAO) micelles, the stability showed a maximum as the degree of ionization increased. The analysis suggests an attractive interaction between the cationic and nonionic head groups in DDAO mixed micelles. Values of (B_{1}) on other ionic/ nonionic pairs were evaluated and discussed. Relation between the present analysis and the regular solution approach is discussed. ΰ€i 1995 Academic Press. tre.