Abstract: Small angle X-ray scattering curves have been obtained for a series of Na Wyoming Bentonite clay samples containing 10% clay by weight and NaPO3 in concentrations ranging from 0 to 100 meq/l. From the scattering data, the relative probability of spacings between parallel clay platelets was computed. For the sample containing no NaPO3, the probability distribution showed a relatively broad maximum at an interparticle spacing of about 180 Å. As the concentration of NaPO3 increased, the maximum became sharper and occurred for smaller interparticle distances. At NaPO3 concentrations between 25 and 100 meq/l, the position d of the maximum was given approximately by the equation d=21+18.4c−1/2, where d is in angstroms, and c is the NaPO3 concentration in eq/l. The similarity of this relation to the dependence of d on the concentration of NaCl (Norrish and Rausell-Colom, 1963; Norrish, 1954) suggests that the interparticle spacing depends primarily on the sodium ion concentration and not on the concentration of the anion. The value of d appears to be independent of whether the gel was prepared by the method of Norrish and Rausell-Colom, in which a dried flake was allowed to come to equilibrium with an electrolyte solution, or whether, as in this investigation, the gel was obtained by centrifuging a dilute suspension. Since the Na ions act to reduce the double-layer repulsion between platelets, while the anions tend to be adsorbed on the platelet edges and thus reduce the edge-to-face linkages (H. van Olphen, 1962), the value of the most probable interpartiele distance appears to be determined primarily by the magnitude of the double-layer repulsion, even though other properties of the clay gels, such as the rheological behavior, are governed mainly by edge-to-face attractions.