This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SM-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30-40 degrees C) and high temperatures (100-120 degrees C). DMFC power densities were about 140 mW cm(-2) at 100 degrees C with the bare SPSf-60 membrane and 180 mW cm(-2) at 120 degrees C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm(-2) at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices. (c) 2008 Elsevier B.V. All rights reserved.