The effect of K+ on freshly precipitated BaSO4 is seen to increase the area of the crystals. The exchange rate of Ba2+ with these crystals is found to be proportional to this increased area. The extent of234Th precipitation is found to be, increased more than can be accounted for by the area change, and it is suggested that the concentration of Th4+ adsorption sites increases as more K+ is incorporated into the lattice. Activation energy measurements show the reaction mechanisms to be unchanged by the presence of K+. Values of 11.4 and 24.4 Kcal·mol-1 for the Ba2+ and Th4+ reactions, respectively, are consistent with an activation process involving loss of water of hydration.