A specialized autoclave has been developed for the measurement of crystal growth and dissolution rates in aqueous salt solutions at temperatures up to 140°C. Contamination during crystal growth or dissolution has been eliminated by having the solution in contact only with the chemically inert materialsglass, Kel-F and Teflon.Provision has been made for sampling both solution and solid phases during the reactions and the utility of the apparatus is demonstrated by measuring reproducibly, the crystal growth rate of calcium sulphate dihydrate (CaS04.2H20) in aqueous solutions at 100°C.