Nanocrystalline spinel LiMn2O4 has been prepared and treatment of LiMn2O4 with dilute nitric acid solution resulted in the delithiation of the framework, while maintaining the spinel structure, lambda-MnO2 center dot LiMn2O4 is not a catalyst for water oxidation. Upon removal of the lithium, the cubical Mn4O4 cores become active sites for oxidizing water to molecular oxygen, which was investigated with the photochemical [Ru2+(2,2'-bpy)(3)]/persulfate system at pH 5.8. The nanosize lambda-MnO2 obtained from the nanocrystalline LiMn2O4, which was synthesized by the citrate route, shows a significantly higher water oxidation catalytic activity (Turnover Frequency: 3 x 10(-5) MOl O-2/s/mol Mn) than that obtained via solid state reaction with micrometer and irregular particle sizes (Turnover Frequency: 5 x 10(-6) mol O-2/s/mol Mn).