A reverse-micelles procedure followed by annealing is developed to prepare Co3O4 materials with controlled particle size and microstructure. The single-phase products occur as submicrometric particles that interconnect into larger aggregates with spherical shape and no porosity. Two types of electrochemical response are found in lithium cells depending on annealing time and temperature. Reversible capacities of up to ca. 800 mAh g(-1) are observed in intermediate annealing, while lower capacities and better capacity retention are found for short treatments at 600 degrees C. These differences are related to the changes in charge-transfer resistance. (C) 2008 The Electrochemical Society.