The mixed micellization between the anionic dimeric surfactant Dim1 (disodium 1,11-didecyl-3,6,9-trioxaundecane 1,11-disulfate) and the two nonionic surfactants C12E5 and C12E8 (penta-and octaoxyethylene monododecyl ethers, respectively) has been investigated. The cmc of the mixtures has been measured by fluorescence probing, using the fluorescent probe pyrene. Its variation with composition revealed synergism in micelle formation. The micelle aggregation (N) numbers have been measured using time-resolved fluorescence quenching at a total surfactant concentration of about 70 mM and at 25, 40, and 55 degrees C. The differences in total aggregation number of the mixed micelles in the two systems reflect (i) the higher aggregation number of C12E5 micelles with respect to C12E8 and Dim1 micelles; (ii) the higher cloud temperature of C12E8 with respect to C12E5; and (iii) the opposite temperature dependences of N for Dim1 and nonionic surfactants micelles. The results show nonideal mixing behavior with a shallow minimum in the variation of the mixed micelle aggregation number with the mixture composition at a Dim1 mole fraction around 0.5.