A statistical associating fluid theory (SAFT) equation of state, incorporated with mean spherical approximation (MSA), has been established to calculate the activity coefficients of surfactants in aqueous solutions. This equation describes molecules as chains of hard sphere segments, and includes the contributions from the hard sphere-hard sphere interaction, the dipole-dipole interaction, the chain formation, the Lennard-Jones interaction, and the charge-charge interaction. The critical micellar concentrations of single aqueous ionic surfactant systems are correlated and predicted. The results are satisfactory, and the adjustable parameters in the model have obvious physical meaning.