Open-framework zinc phosphate (NMe4)(ZnP2O8H3) undergoes irreversible phase transformation. Structural transformation with α (NMe4·Zn[HPO4][H2PO4] the low-temperature phase) and β (NMe4·ZnH3[PO4]2 the high-temperature phase) (Tc = 149 °C) and conduction properties were investigated by single-crystal X-ray diffraction, differential scanning calorimetry, and alternating current (ac) impedance. The open-framework material was sensitive to humidity and β proton conductivity was higher than 10-2 S cm-1 at room temperature and 98% relative humidity (RH). Given that the high proton conductivity of the open-framework material can compete with that of many advanced proton conductors based on metal-organic frameworks (MOFs), it has broad application prospects in various electrochemical devices.