The determination is carried out as follows: Potentiometric determination of the chloride or bromide ion activity of a series of solutions of cationic surface active agents the concentration of which cover the expected micellization concentration. Plotting the graph of potential as a function of the logarithm of the concentration; the c.m.c. corresponds to the singularity on this curve. The method is applicable to purified cationic surface active agents (hydrochlorides and/or hydrobromides), which are soluble in water and which have a Krafft temperature below 60 degree Celsius.