Microcalorimetric, open‐circuit potential, and capacity measurements were made to investigate the effects of hydrogen pressure and concentration on the self‐discharge characteristics of nickel hydroxide electrodes at 25°C. By carrying out microcalorimetric experiments with powders of the constituents of the nickel hydroxide electrode, i.e., the active material and substrate, separately and combined, it was shown that hydrogen oxidation occurs predominantly on the active material with simultaneous reduction of the oxide. The results also show that the rate of heat generation by powders, immersed in , increases with the increase of hydrogen pressure; this result explains the shift of the open‐circuit potential of the nickel hydroxide electrode in a negative direction, this shift occurring much more rapidly in a hydrogen environment than in an inert atmosphere. From the microcalorimetric data, it was deduced that the rate of heat generation corresponds to a capacity loss of about 3.1% per day when the hydrogen pressure is about 50 atm. Nickel hydroxide electrodes, at high pressures of hydrogen (e.g., 700 psi), lose capacity more rapidly in a concentration of 23% than in 26% or 31%. However, no significant effects of concentration on self‐discharge rates of the nickel hydroxide electrode were observed at a lower pressure of hydrogen (150 psi).