The absorption of hydrogen by RTx alloys (r=Ce, La, Nd, Pr, and MM (mischmetal); T=transition metals, chiefly CO, Ni, and Fe) has been studied by the Bureau of Mines with the object of developing a suitable alloy absorbent for hydrogen storage at 25 deg C and a pressure range of 1 to 10 atm. The ability to absorb and desorb hydrogen reversibly under these conditions (that is, the working capacity) reached a maximum with the composition RT5. LaNi5 exhibited the highest capacity. Studies with MMCo5 showed that variations in individual rare earth contents in mischmetal had little effect upon the absorption properties of the alloys, but partial substitution of cobalt by other transition metals had a substantial effect. Interstitial oxygen and carbon in MMCo5 considerably reduced both the hydrogen absorption and the working capacity.