The Mnary system Na,S-NaOH was Investigated by dlffwentlal thermal analysis, and the room-temperature phases were determined by X-ray dlffractlon data. This system Is a sknple eutectic system. The eutectic temperature and compodtlon are 296 f 1 OC and 0.05 mole fractlon NazS. The polymorphlc tranotormatlon In NaOH occurred at 291 OC. The lowerlng of thls transltlon from 293.1 OC, the accepted value, Is belleved to be due to reddual moisture In the NaOH. No evldence for the two compounds Na,S-NaOH and Na2S-2NaOH, reported In the Ilterature, was observed. Mlxtures of these comporttlons gave thermal arrests at eutectic and transformation temperature. X-ray dlffractlon patterns showed llnes whlch could be accounted for on the basts of known patterns for cuMc Na,S and orthorhomblc NaOH. Ltterature data for the NaOH-Na,C03 system were revlewed and employed to construct the phase dlagram. 'Thir h also a dmple eutectlc system wlth a eutectlc temperature and composltlon of 288 f 1.0 OC and 92.0 f 0.1 mol % NaOH.