The cause of the memory effect observed in alkaline-type rechargeable batteries such as nickel–cadmium and nickel–hydrogen batteries was studied using a positive capacity-limited nickel–cadmium cell and AAA-type commercially available nickel–cadmium and nickel–hydrogen batteries. From the X-ray diffraction (XRD) analysis, γ-NiOOH was observed on the nickel electrode in a charged state after repeating shallow discharge cycling of the cells or overcharging. This γ-NiOOH is initially formed at the collector side of the electrode and it then grows to the solution side during shallow discharge cycling. When the amount of γ-NiOOH formed is small, only β-NiOOH can be detected by conventional XRD, even when the memory effect is observed. In this case, γ-NiOOH can be detected by shaving the surface of the electrode, using an emery paper to remove the β-NiOOH covering. This γ-NiOOH disappeared within a few cycles of the normal charge–discharge cycling and the memory effect disappeared. It is concluded that the cause of the memory effect is mainly due to the formation of γ-NiOOH.