Abstract: The consequences of a nonhomogeneous charge distribution in clay systems are first examined. The mean ionic activity is shown to be a geometrical rather than an arithmetical average. Potentiometric methods of measurement, with (quasithermodynamic), and without (thermodynamic) liquid junctions are compared. The latter give chemical potentials of soluble molecular species or ratios of mean activities; neither of these affords a characterization of single cations in relation to clay surfaces. The quasithermodynamic method, like conductance measurements, affords data which can be interpreted directly in terms of single ions. The two are compared for dilute and more concentrated clay systems by using the ratio: “Cation conductivity/total conductivity of clay systems.” For dilute systems this ratio varies slowly and smoothly with concentration and with degree of saturation. High concentrations give an abnormally high value. Cataphoresis results are shown to be in agreement with conductance and potentiometric data for dilute clay systems.