The synergistic effects in mixed surfactant systems containing ionic surfactants with the same electrical charge and different hydrocarbon tails and ionic-zwitterionic surfactants are studied. Results presented in the work show that the synergism is mainly due to entropic free energy contributions. In mixtures with two ionic components the synergism increases when the difference between the cmc of pure components increases. This is because the addition of an ionic surfactant with high cmc increases the electrolyte concentration reducing the chemical potential of surfactant in the mixed aggregates, and as a consequence reduces the cmc of aggregates. In the case of ionic-zwitterionic surfactants the synergism is only observed when salts are added. The synergistic effects are also of electrostatic origin; in this case, the synergism is due to the release of counterions from the diffuse layer to the solvent.