The mixed aggregation process of two cationic surfactants with the same number of carbon atoms on the hydrophobic tail and different polar heads, dodecylethyldimethylammonium bromide (D(12)EDMAB) and dodecylpyridinium chloride (D12PyC), has been characterized in aqueous solution by conductivity, speed of sound, density, and static and dynamic light scattering measurements at 298.15 K. From these data, the monomeric and micellar phases of the mixed aggregates were fully analyzed through the determination of the total (cmc*) and partial (cmc(1)* and cmc(2)*) critical micellar concentrations, the dissociation degree of the mixed micelle (beta), the total (N*) and partial (N-1* and N-2*) aggregation numbers, apparent molar volumes (V-phi), and isentropic compressibilities (K-s,K-phi) and hydration numbers (n(h)), as well as the corresponding changes in the latest properties due to the mixed aggregation process. The marked differences among the micelles of the pure surfactants studied in this work result in clear deviations from ideal behavior when the mixed aggregation process occurs. These experimental evidences have also been analyzed in terms of several theoretical models.