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, gamma -NiOOH was observed on the nickel electrode in a charged state after repeating shallow discharge cycling of the cells or overcharging. This gamma -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 gamma -NiOOH formed is small, only beta -NiOOH can be detected by conventional XRD, even when the memory effect is observed. In this case, gamma -NiOOH can be detected by shaving the surface of the electrode, using an emery paper to remove the beta -NiOOH covering. This gamma -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 gamma -NiOOH. (C) 2001 Elsevier Science B.V. All rights reserved.