Phosphorylated phenol-formaldehyde (PPF) resins were synthesized and investigated as the acidic components of proton conducting polymer electrolytes. The synthesis of PPF resins was carried out by melt polycondensation of monophenyl phosphate and formaldehyde (in the form of trioxane). The structure and molecular weight characterization of PPF resins were performed employing H-1-, C-13- and P-31-NMR spectroscopy, Fast Atom Bombardment mass spectrometry and elemental analysis. PPF resins may be cured with an excess of formaldehyde. Depending on the curing conditions, products of different crosslinking density (expressed by equilibrium swelling ratio) and total ion-exchange capacity up to 9 mequiv. g(-1) may be obtained. The soluble and crosslinked PPF resins were examined from the viewpoint of conducting properties and hydrolytical stability. Crosslinked PPF resins embedded in poly(vinylidene fluoride) binder form ion-exchange membranes of ambient conductivities above 0.05 S cm(-1). These composite membranes have been investigated in a methanol fuel cell and showed stable performance during several hours of cell operation. (c) 2006 Elsevier B.V. All rights reserved..