The kinetic exchange SO42- ions with CO32- ions in natural strontium sulfate (celestite) single crystals under alkaline hydrothermal conditions was investigated. The Experiments were performed at several temperatures ranging from 423 to 523 K for different reaction times between 1 and 96 h with molar ratios of CO32-/SO42- = 1, 5 and 10. Structural characterization of partially and completely converted strontianite (SrCO3) crystals was conducted by XRD, FTIR and SEM. X-ray diffraction results showed that under hydrothermal conditions, the exchange of CO32- ions in celestite was complete at 523 K in a Na2CO3 solution with a molar ratio CO32-/SO42- = 10 for 96 h. Based on the experimental observations, a mechanism for the conversion of celestite, under hydrothermal conditions is proposed. As the reaction proceeds, a porous layer of strontium carbonate forms on the celestite surface. The reaction rate is determined by the diffusion of sulfate ions through the porous layer. The apparent activation energy for the conversion from celestite to strontium carbonate is 21.6 kJ mol(-1). (c) 2007 Elsevier B.V. All rights reserved.