The effect of temperature on the cathode potential during nickel plating was studied at current densities ranging from 0.4 to 100 ma/cm2 and at temperatures from 20° to 90°C. The cathode potential‐temperature relation is linear with a break in the slope at about −0.42 volt. The extrapolated curves above −0.42 volt converge at about 0 volts and 165°C. An empirical equation was developed from the data: . The observed change in cathode potential with current density is attributed to a hydrogen overvoltage which is controlled by the diffusion of hydrogen ions through a space‐charged layer of nickel ions at the cathode surface. The equation does not apply at current densities below about 0.4 ma/cm2. A hysteresis effect was observed in Ec‐T curves using newly polished electrodes when the data were obtained first by decreasing and then increasing the temperature. Deviations from the empirical equation were attributed to hydrogen adsorption.