The temperature-programmed vaporization of crystal water in vacuum was studied for a thin film of Ca(NO3) . nH2O prepared on a nichrome ribbon heater. The mass-spectrometric signal intensity of vaporized water measured as a function of the film temperature reveals a spectrum consisting of two sharp peaks, in remarkable contrast to the broad spectra observed for other kinds of hydrated inorganic salts. The low-temperature peak exhibits normal kinetic features which seem to be consistent with direct emission of water molecules from the crystalline lattice of the hydrated salt. However, the high-temperature peak appears in an explosive manner, and its intensity relative to the low-temperature peak increases with increasing film thickness or with increasing heating rate. These features of the high-temperature peak suggest the occurrence of a kinetic phase transition of the hydrated salt from a crystalline state to a non-crystalline state, rather than step-wise dehydration of the crystalline salt.