PEO sulfonic acids with [M(W)] in the range 446-4246 have been prepared. Mechanically stable polyelectrolyte films containing high molar mass PEO and PEO sulfonic acids were prepared. The PEO sulfonic acids and the polyelectrolyte films were examined by thermal analysis, optical microscopy, Raman spectroscopy, and impedance spectroscopy. While the low molar mass PEO sulfonic acids were completely amorphous, sulfonic acids with [M(W)] greater than or equal to 1246 show considerable crystallinity. Experimental data indicate aggregation of the low molar mass PEO sulfonic acids through hydrogen bonds. The PEO sulfonic acids are miscible with high molar mass PEO and form free standing polyelectrolyte films. The PEO sulfonic acids with the lowest molar masses have a plasticizing effect on the high molar mass PEG. The crystallinity of the films decreased as the concentration of sulfonic acid increased. The films are stable at RH less than or equal to 75%, and for some mixtures protonic conductivities of 10(-3) S cm(-1) at room temperature were reached.