The hydrothermal-electrochemical synthesis route has been used to prepare a variety of MnO2 compounds with the alpha, beta and gamma structure types as well as intimate mixtures of these structure types. The gamma-MnO2 compounds can be synthesized with varying amounts of pyrolusite intergrowth, and low occurrence of microtwinning defects. The influence of the synthesis parameters (temperature, pH, applied current density, composition of the synthesis solution) on the type of structure obtained are discussed. The morphology of some of the compounds obtained is presented. A lithium insertion study compares the behavior of alpha- and gamma-MnO2. The alpha-MnO2 shows a very stable capacity of 145 mAh/g at a discharge rate of C/10. (C) 2003 Elsevier Science B.V. All rights reserved.