Electrical conductivity studies have been performed of solid acid phosphites MH(PO3H) with M = Li+, Na+, K+, Rb+, Cs+, NH4+. Superprotonic conductivity following a phase transition in the temperature range from 120 to 190 °C is observed for the monoclinic forms, including Na+, K+, Rb+, Cs+ and NH4+ derivatives. At temperatures beyond the superprotonic transition temperature, thermal decomposition occurs in the case of the Na+, Rb+, and NH4+ salts. No superprotonic phase transition is observed for orthorhombic LiH(PO3H). The magnitude of the superprotonic conductivity of KH(PO3H) and CsH(PO3H), which is known to be stable under hydrogen atmospheres from literature, amounts to 4.2 · 10− 3 Ω− 1 cm− 1 (at 140 °C) and 3 · 10− 3 Ω− 1 cm− 1 (at 160 °C), respectively, which values are comparable to that of well known CsHSO4.