Water desorbing from cordierite-type glass powders, with a narrow particle size distribution, was studied by Knudsen effusion mass spectrometry. The thermal desorption spectra show both low temperature (300-800 K) and high temperature (800-1500 K) peaks that represent surface and bulk effects, respectively. Carbon dioxide desorbs in two low temperature peaks below 1000 K; the lower peak is attributed to the decomposition of a bicarbonate. However, above the glass transition temperature (approximately 1050 K), water diffusing from the bulk desorbs in a first-order process with an activation energy of 260 kJ/ mol. During the onset of bulk water desorption, the rate is dominated by bulk diffusion of water. All these processes are related to the glass particle size. Smaller particles store and, consequently, desorb more 'surface' water. As the particle size gets larger, the temperature increases, at which the maximum in the bulk water desorption rate occurs. This change corresponds to an increase in the frequency factor, which correlates inversely to the mean particle size diameter.