An apparatus is described allowing the study of controlled rate thermodesorption (CRTD) in the temperature range from 163 to 473 K and in the residual pressure region around 10(-3) mbar. This apparatus is designed with the aim to increase the sensitivity in the estimation of the specific surface area with respect to traditional adsorption experiments. The thermodesorption of water from a model mesoporous MCM-41 sample is studied using this apparatus. The thermodesorption curve obtained is related to the mesopore size distribution via an expression based on the Kelvin law. These results compare well with those obtained by adsorption gravimetry of water at 303 K on the same MCM-41 sample.