A heterogeneous isotopic exchange reaction of strontium polymolybdate in strontium chloride solution was studied using Sr-90 as a tracer. The effects of low and high strontium chloride concentration on the rate and mechanism of the isotopic exchange reaction were investigated. It was found that, at high concentrations, the rate is independent of strontium concentration, but, at low concentrations, the rate is proportional to the strontium concentration, These results support a hypothesis that, at low concentrations, the rate is controlled by film diffusion, whereas at high concentrations it is controlled by particle diffusion. Experiments were performed at 293, 303 and 313 K. Activation energy of isotopic exchange reaction and thermodynamic parameters DeltaH*, DeltaS* and DeltaG* were calculated using the Arrhenius and Eyring equations. The results also indicated that recrystallization is a predominant factor in the present exchange reaction.(C) 2002 Elsevier Science Ltd. All rights reserved.