We synthesized mesoporous/crystalline composite with a mass ratio of 1: 2 (mesoporous: crystalline), and the resulting mesoporous tin phosphate has different pore size depending on the alkyl chain length (n = 11, 13, 15, and 17) of the surfactant CH3 (CH2)(n)N (CH3)(3)Br. Because the crystalline phase (Sn2P2O7) to mesoporous phase mass ratio is fixed, a decrease in the capacity retention is mainly attributed to the deterioration of the mesophase. The mesoporous phase with smaller pore size causes higher capacity retention, with n = 11 showing the highest capacity retention corresponding to 82 %. This decreased to 67, 63, and 59 % with increasing pore size, n = 13, 15, and 17, respectively. (c) 2005 The Electrochemical Society. All rights reserved.