In a series of test tubes, increasing amounts of pure cetyltrimethyl ammonium bromide (Lissolamine A) are added to a dilute clay suspension containing a known amount of clay. Visual examination of the contents of the tubes shows a stability minimum to exist at a certain concentration of the bromide. It may be assumed that in this stability minimum the clay micelles are neutralized by quantitative exchange of the exchangeable ions of the clay by an equivalent amount of strongly adsorbed organic cations of the bromide. Consequently the b.e.c, of the clay may be calculated from the concentration of the cation-active compound at the stability minimum. Since, however, by visual examination the stability minimum cannot be distinguished sharply enough, a better criterion had to be developed. As there are reasons to assume that the neutralized micelle will act as an emulsifier for oil-water systems, an emulsification test was expected to offer a way out. It was indeed found that on shaking the contents of the tubes with gas-oil, a rather stable emulsion was formed at a certain critical concentration of the cation active compound. Taking this concentration as an indication of quantitative exchange, the b.e.c, of the micelle may be easily calculated. Results of preliminary experiments are in good agreement with those obtained by application of conventional methods.