NOVELTY - Hydrogen generator system comprises a hydride/hydrate combination, where the hydride/hydrate combination comprises a hydride that is physically mixed with a ratio of hydrate, water, and catalyst in such proportions that the system temperature is kept constant, or nearly constant, or at or near a selected phase transition temperature of the hydrate of 30-90 degrees C during generation of hydrogen gas by hydrolysis of the hydride. The starting proportions of the hydride and the hydrate are determined based on calculations that take into account the heat of formation of liberated water molecules.    USE - The hydrogen generator system is useful: for generating hydrogen gas from a hydrogen source composition at a predefined temperature range (claimed); and in fuel cells, combustion engines, and for military applications including an expendable communications relay.    ADVANTAGE - The hydrogen generator system accurately balances heat released by the hydride during hydrolysis and heat absorbed by the hydrate as the hydrate undergoes a phase transition to a less hydrated or to an anhydrous form.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) generating (M1) hydrogen gas from a hydrogen source composition at a predefined temperature range, comprising selecting at least one hydride and at least one hydrate, determining the number of water molecules that can be removed from the hydrate at or below the predefined temperature range, calculating the total heat of dehydration involved in removing the water molecules from the hydrate at or below the predefined temperature range, where the calculation of the total heat of dehydration takes into account the heat of formation of liberated water molecules, calculating the total heat of hydrolysis involved in removing the hydrogen from the hydride during hydrolysis, calculating the relative amounts of reactant hydride and reactant hydrate needed to balance the total heat evolved by the dehydration reaction and total heat used by the hydrolysis reaction, combining the hydride and hydrate in the relative amounts calculated for balancing the heats of dehydration and hydrolysis, and generating hydrogen gas; and    (2) generating (M2) hydrogen gas from a hydrogen source composition at a predefined temperature range, comprising (a) selecting a hydride/hydrate combination, (b) determining the number of waters of hydration associated with the hydrate, (c) determining a dehydration temperature of a given water of hydration of the hydrate and determining if the corresponding dehydration temperature is within the predefined temperature range, (d) repeating step (c) for every water of hydration of the hydrate, (e) repeating steps (a)-(d) as above per se if none of the dehydration temperatures determined in steps (c)-(d) are within the predefined temperature range, (f) selecting one of the waters of hydration having a selected dehydration temperature within the predefined temperature range, (g) obtaining a heat of reaction for (a1) the dehydration of the selected water of hydration of the hydrate and for (b1) the formation of liberated waters released by the dehydration of the selected water of hydration, (h) calculating the sum of the heats of reaction (a1) and (b1) and providing the sum as a positive enthalpy, (i) obtaining a heat of reaction for the hydrolysis of the hydride to generate hydrogen gas, (j) calculating the fraction of moles of hydrate reactant needed to counterbalance the heat of hydrolysis from a mole of hydride reactant, and (k) combining the fraction of moles of hydrate per 1 mole of hydride into a mixture to generate hydrogen gas at the selected dehydration temperature.