For solutions containing a 1 : 1 ionic surfactant plus a 1 : 1 electrolyte with a common counterion the expression α²_±=C₃[C₃+m₁+α(C₁–m₁)]γ² is used to estimate the effective micellar degree of dissociation α from experimental values of a_±. The mean ionic activity of supporting electrolyte in situations where m₁C₃C₁(C₃, C₁ and m₁ denote concentrations of added salt, total ionic surfactant and ionic-surfactant monomer, respectively, and γ is an appropriate activity coefficient). The method is free from uncertainties due to liquid-junction effects. For pure ionic surfactants α is found to depend only weakly on C₁ and agrees well with values obtained from the effect of salt on the c.m.c. For mixtures of ionic + non-ionic surfactants 1/α is found to vary linearly with micellar mole fraction of the non-ionic x_N and aproach unity as x_N→ 1. The implications of this finding on the validity of a recent theory describing the c.m.c. of such mixtures is discussed.