The-phase is formed by the absorption of small amounts of hydrogen, the maximum composition is about PdHo. os at room temperature, and it has a lattice parameter only slightly larger than that of purepalladium (3.889 Á.). As hydrogen is further absorbed, the «-phase becomes unstable andthe lattice suddenly expands toabout 4.018 A.(PdH0. e) to form the/3-phase. Additional hydrogen uptake is accom-panied by a gradual expansion of the «-phase lattice. X-Ray diffraction studies of the Pd-H2 system have, for the most part, been performed at room temperature on samples preparedeither electrolytically or at elevated temperatures. Either procedure involves some uncertainty as to what may occur in the specimen between thetime it is charged and thetime the X-ray exposure is made. Recognizing this problem, Owen and Jones7 and Owen and Williams8 charged the specimen from the gaseous phaseand made the X-ray exposures while the specimen was maintained at a definite temperature and constant hydrogen pressure. The present investigation is an extension of the investigation of Owen and Williams to higher temperatures and pressures. Six evolution iso-therms, from 206 to 346, have been traced from 33 atm. to zero pressure.