Chloride ion effects on the contents of hydrogen absorbed in nickel deposits used as an electrode during cathodic electrolysis of H2O were investigated. Time-variation of the internal strain occurring in nickel electrode during electrolysis was measured in situ using a resistance wiretype strain gauge placed on the reverse side of the copper substrate. Based on the buoyancy produced by the hydrogen gas bubble, the amounts of hydrogen absorbed into the nickel electrode and evolved during electrolysis were evaluated quantitatively using an electric balance. The current density for nickel electroplating was 30 mA/cm2. The conditions for cathodic electrolysis of H2O were electric charges of 0.20 C and current density of 2 mA/cm2.The tensile stress in nickel deposits was enhanced greatly with increased concentration of chloride ions in the electroplating baths beyond 33 mol% by [Cl-]×100(/[SO42-]+[Cl-]).However, during cathodic electrolysis of H2O on the nickel deposits used as the cathode elec trode, the buoyancy attributable to hydrogen gas bubbles started after the elapsed time of 19 s from application of electrolysis, which resulted from hydrogen absorption into the nickel electrode. Gas bubble formation continued for some time after the current was turned off, which was associated with hydrogen coming from the electrode. Hydrogen contents absorbed in the nickel electrode increased linearly with increasing concentration of chloride ions described as [Cl-]×100(/[SO42-]+[Cl-])in the acid electrolyte.