Polymer blends based on poly(vinyl alcohol) (PVOH), hypophosphorous acid (H3PO2) and water behave as solid protonic conductors. The ionic conductivity at room temperature depends strongly on OH/P ([number of mols of vinyl alcohol monomeric units]/[number of mols of H3PO2]) ratios, x, showing a sensitive variation between 10 (-6) and 10(-1) S cm(-1), as the acid concentration is increased. Using the membranes as electrolyte separator in a fuel cell, voltages up to 726 mV have been obtained. Using high-resolution AC calorimetry and differential scanning calorimetry, we studied the phase behavior of the blends. We found that, independent of x, the glass transition (T-g) is about 167 K, compared with that of PVOH/H2O at 298 K and pure PVOH at 318 K. However, the step change at the glass transition and other two thermal events (peaks) at higher temperatures (at about 220 and 255 K, respectively) representing the cold crystallization and the corresponding crystalline melting of the samples, increase with increasing acid content in the samples. These results are consistent with the interpretation of a two-phase mixture in the blends: one of them with constant composition PVOH/H3PO2/H2O and a separate second phase with composition H3PO2/H2O.