Kinetics of water sorption on loose grains of composite sorbent CaCl2 confined to mesoporous silica (SWS-1L) was measured at T=33-69 degrees C and (P)(H2O) = 8-70 rnbar over water uptake range 0-0.47 g/g for various particle sizes R-p (between 0.355 and 1.4 mm). The measurements were performed in a constant pressure unit based on a CAHN microbalance under isothermal external conditions. The results obtained evidence an enhancement of the sorption rate and apparent diffusion constant with the decrease in the particle size (approximately as R-p(-2) at R-p > 0.71 mm). Contribution of thermal effects was found for water sorption on smaller SWS particles (0.355-0.425 mm), which decreases the sorption rate. The apparent water diffusivity was found to depend on the local slope of the SWS water sorption isotherm. The pore diffusivity of water in the temperature range 33-69 degrees C was calculated from experimental data D-e = (0.24 +/- 0.12) x 10(-6) m(2)/s that is approximately 10 times lower than the Knudsen pore diffusivity estimated for pores of silica KSK. The possible reasons of the diffusivity reduction are discussed. (c) 2005 Elsevier Ltd. All rights reserved.