The dehydration kinetics of barium chloride dihydrate to the monohydrate and/or anhydrous salt were found to be very sensitive to the temperature, bulk water-vapor pressure, acid sample and particle sizes. Dehydrations were conducted isothermally between 317 and 333 K under either moderate vacuum or water vapor atmospheres ranging from 40 to 6.67 x 10(2) Pa. Particle sizes ranged between 53 and 710 mu m. Activation energies were found to vary with sample size and extent of conversion; apparent values ranging from 92 to 150 kJ mol(-1) were measured. Enthalpy of reaction estimates also varied with sample size and temperature; values ranging from 115.9 to 137.2 kJ mol(-1) were obtained. The sensitivities to the temperature, water vapor pressure, sample and particle size were found to be significantly affected by transient changes in local water vapor pressure caused by inter- and intraparticle diffusional limitations. Possible mechanistic changes with changing water vapor pressure are discussed. Transient increases in water vapor pressure in the interior of large particles caused crystallization of the monohydrate under conditions where it did not otherwise form.