An amorphous Sn-Co alloy film electro-codeposited on rough Cu foil was found in our previous works to self-organize a microisland structure, which is a crucial factor to improve Sn alloy anodes for cyclability. This work focused on the electrochemical properties, film morphology, and phase structure, and the behavior of Co atoms was analyzed by extended X-ray absorption fine structure and magnetic susceptibility measurements. Cycle performance up to 20 cycles, in which the capacity anomalously fluctuated, is discussed based on these results, where the capacity change is explained by the morphological and phase structural changes. The stabilization process of the phase is clarified in connection with formation of ferromagnetic face-centered cubic Co particles. (c) 2006 The Electrochemical Society.