Stresses generated during the hydrogen extraction from and injection into a Ni(OH)2/NiOOH film electrode in 0.1 M KOH solution are analysed by means of a laser beam deflection technique combined with potentiostatic current transient and electrochemical quartz crystal microbalance (EQCM) techniques. From the measured values of the film thickness, the elapsed time for phase boundary movement (PBM) and the potential step given between the hydrogen extraction/injection potential and the plateau potential, the velocity and the mobility of PBM are determined to be 2.3×10−6–5.4×10−6 cm s−1 and 3.5×10−10–2.6×10−9 cm2 s−1 V−1, respectively. From the mass change measured by means of EQCM, it is suggested that, during hydrogen extraction, the PBM continues until the insertion of K+ ions begins to occur simultaneously with the desertion of such neutral species as H2O molecules or KOH molecules. During hydrogen injection, the PBM is accompanied by the extraction of K+ ions, followed by the insertion of neutral species. From the mass change transient and the deflection transient, simultaneously measured with the current transient, it is suggested that the sharp rise of tensile and compressive deflections in the initial stage is traced back to the PBM. The following relaxation of tensile and compressive deflections is attributed to the extraction/insertion of neutral species. Most of the stresses developed during the hydrogen extraction and injection originate mainly from the PBM, not the extraction/insertion of K+ ions and neutral species.