Phosphorous acid reacts with the superacidic system HF/SbF5 with formation of HPF4, water, and oxonium hexafluoroantimonate, which crystallizes as H5O2+SbF6- in the triclinic space group P (1) over bar with a = 6.492(6) Angstrom, b = 7.469(8) Angstrom, c = 8.095(6) Angstrom, alpha = 116.17(7)degrees, beta = 93.90(7)degrees, and gamma = 112.34(7)degrees with two formula units per unit cell. The unit cell contains two H5O2+ ions of approximate symmetry C-2h With an intramolecular O ... O distance of 2.418(8) or 2.40(1) Angstrom, respectively. The reaction is discussed with regard to the formation of protonated species in various superacidic media. In addition the crystal structure of Me4N+HPF5- which is formed by the reaction of HPF4 with Me4N+F- is reported. The salt crystallizes in the tetragonal space group P4/nmm with a = 8.564(5) Angstrom and c = 5.687(2) Angstrom, with two formula units per unit cell. The P-F distances of P-F-eq = 1.595(6) Angstrom and P-F-ax = 1.594(9) Angstrom for the psi-octahedral HPF5- show no significant interaction exercised by hydrogen on the cis- and trans-fluorine.