Nafion/poly(phenylmethyl silsequioxane) (PPSQ)/calcium hydroxyphosphate (CHP) and Nafion/PPSQ composite membranes are fabricated using homogeneous dispersive mixing and a solvent casting method for direct dimethyl ether fuel cell (DDMEFC) applications for use above 100 degrees C. The recast composite membranes were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and thermogravimetry (TGA). The results showed that PPSQ and CHP are compatible with the Nafion membrane and improve the initial degradation temperature of the Nafion membrane. In addition, the crystallinity calculated by a Gaussian peak deconvolution technique increases as the PPSQ and CHP increase in composite membranes. This composite membrane showed less fuel crossover compared to both the recast Nafion membrane and Nafion 115. This is likely to be due to the high crystallinity and low water uptake content in the composite membranes. On the other hand, the fuel crossover of the composite membrane impregnating CHP increased as the CHP content increased. This is related to the hydrophilic character of CHP. In addition, the proton conductivity of the composite membranes containing PPSQ and CHP appeared to be higher that of the Nafion membrane at a high temperature. Nafion/PPSQ/CHP and Nafion/PPSQ composite membranes showed better cell performances compared to Nafion 115 at temperatures ranging from 100 degrees C to 120 degrees C and at pressures from 1 atm to 2 atm.