H-1 NMR measurements on a new family of mesoporous molecular sieves, designated MCM-41, saturated with water show abrupt changes in the signal intensity of the water signal at specific temperatures, denoted as transition temperatures. A model equation is presented describing the intensity vs temperature behavior. Similar transition phenomena are also observed in line width vs temperature. A linear relationship between the spin-lattice relaxation rate (at -10 degrees C) and the first transition temperature, as determined from the intensity measurements, is found. This behavior is expected if it is assumed that the transition temperature can be predicted by Kelvin’s equation and that the spin-lattice relaxation rate is proportional to the inverse of the pore radius, as previously reported in the literature.