Composite membranes composed of modified S or Si-zirconium phosphate (ZrP), porous polytetra-fluoethylene (PTFE) and, glycerol (GLY) were synthesized in this work. ZrP was precipitated via the insitu reaction of zirconium oxychloride (ZrOCl2) with phosphoric acid (H3PO4). Silicic acid and sulphuric acid were introduced as additives to phosphoric acid with variable Si/P or S/P mass ratios in the acid solution/suspension. The modified membranes were investigated as electrolytes for direct hydrocarbon fuel cells operating at temperatures similar to 200 degrees C. The present work shows that adding a small amount of silicic acid to phosphoric acid enhanced the proton conductivity, i.e. by having a Si/P mass ratio of (0.01) in the acid solution/suspension, the Si-ZrP/PTFE/GLY membrane conductivity was 0.073 S cm(-1). This value approached the conductivity of Nafion (0.1 S cm(-1)). The results also showed that adding sulphuric acid to phosphoric acid led to a significant decrease in the membrane's proton conductivity. SEM analysis results showed a porous structure of the Si-ZrP modified membranes. This porous structure combined with the high proton conductivity reported, would make them good candidates for catalyst layer supports in fuel cell applications. (C) 2012 Elsevier B.V. All rights reserved.