The aim of this study was to apply the regular solution theory of mixed micelles to gain new insights on the drug release mechanism, when using catanionic mixtures as a method of obtaining prolonged release from gels. Synergistic effects were investigated at equilibrium and quantified in terms of regular solution theory interaction parameters. The drug release from catanionic aggregates was studied both in a polymer free environment, using dialysis membranes, and in gels, using a modified USP paddle method. The drug release kinetics was modelled theoretically by combining the regular solution theory with Fick's diffusion laws assuming a contribution to the transport only from monomeric species (stationary aggregates). The theoretical predictions were found to be in reasonably good agreement with experiments. An analysis of the calculated distribution of species between aggregated and monomeric states was shown to provide further insights into the release mechanism.