A process for producing an ion exchange membrane involves melt-processing a mixture of a perfluorosulfonic acid ionomer in its acid form and a specific azole additive. The additive may be a triazole, alkyl triazole, vinyl triazole, fluoro-alkyl triazole, fluoro-vinyl triazole, pyrazole, alkyl pyrazole, vinyl pyrazole, fluoro-alkyl pyrazole, fluoro-vinyl pyrazole, benzimidazole, alkyl benzimidazole, vinyl benzimidazole, fluoro-alkyl benzimidazole, fluoro-vinyl benzimidazole or any mixture thereof to form a film having a thickness of from 3 to 200 microns. Ion exchange membranes so produced have reduced in-plane-swelling, improved dimensional stability and mechanical properties, and are useful as electrolytes in proton exchange membrane fuel cells.