1. Interaction in the NaAlH4 — THF system has been investigated in the temperature range −110 to 25° by the differential thermal, visual polythermal, and isothermal solubility methods. 2. The existence of two compounds has been established, NaAlH4·8THF and NaAlH4·4THF; these melt incongruently at −51 and −32°, respectively. 3. The solubility of NaAlH4 in THF has been determined at the following temperatures: −75, −50, −30, 0, and 25°C; under these conditions its molar and weight concentrations are, respectively, 1.4, 1.05; 11.6 8.95; 19.8, 15.60; 21.06, 16.68; and 22.13, 17.51%.