X-ray diffraction studies (XRD) were carried out for the investigation of the synthesis and electrochemical reduction of lithiated MnO2. The optimal Li:Mn ratio for a heat-treated mixture of LiNO3 + gamma-MnO2 at 370 degrees C (20 h) with a minimum of impurities, such as gamma-beta-MnO2 or spinel, was shown to be equal to 0.33. A combined application of the open-circuit voltage (OCV), slow-scan-rate cyclic voltammetry and XRD measurements was used for the investigation of the intercalation mechanism. The initial compound, Li0.33MnO2, was shown to undergo only one essential reversible transition during its electrochemical reduction to Li0.75MnO2, with a voltage plateau appearing around 3 V. It was conclusively demonstrated that both a thermal synthesis in a certain range of Li:Mn ratio and electrochemical reduction upon cycling result in the phase transition from Li0.3MnO2 to Li0.5MnO2 spinel. The characteristic feature of the latter reduction process is that it is essentially irreversible and occurs in a thin surface layer of the initial material. The formation of this thin layer seems to be responsible for a drop in the capacity of practical electrodes during their charge-discharge cycling. A plausible explanation for this effect is discussed.