The miscibility of poly(vinylphosphonic acid) (PVPA) and poly(ethylene oxide) (PEO) is studied for the first time in this work, and a miscibility diagram is obtained based on thermoanalytical (DSC) and optical microscopy data. On the basis of this knowledge, homogeneous PVPA/PEO blends are prepared as proton-conducting polymer films. The inability of phosphonic acid groups and PEO in the blends is determined by H-1-MAS NMR in temperature-dependent measurements. The effect of composition and the role of PEO on proton conduction are discussed.