The aim of this work is to study the thermodynamics of Sr-Mg vermiculite exchange and the effect of polyvinyl alcohol (PVA) on the interlayer magnesium release from the clay surface. The results indicated that the affinity of adsorption decreases as the unhydrated ionic radius increases. Also it was found that the surface of vermiculite exhibits more selectivity for Sr by treating Mg-vermiculite with PVA than in its absence. With increasing temperature the exchange of Mg by Sr increases. The enthalpy value (Delta H) was positive, i.e. the exchange reaction is endothermic. X-ray diffraction of the clay indicates that the c-spacing of vermiculite increases gradually as the adsorbed amount of Sr ion increases on the vermiculite surface. The activity coefficient ratio of f(Sr)/f(Mg) varies at 20 degrees C from 0.042 at 10% Sr saturation to 0.364 at 90% Sr saturation, i.e. the ratio increased with increasing Sr saturation.