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 (B-1) 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.