The gas-phase chemistry of phosphorus oxoacids, radicals, cations, and cation radicals is investigated by experiment and theory. Vertical neutralization of the stable P(OH)(4)(+) cation forms the P(OH)(4)(.) radical which dissociates exothermically within 4.5 mu s by loss of hydrogen to form phosphoric acid. Collisional reionization of vibrationally excited H3PO4 results in extensive dissociation by losses of hydroxyl groups. Vertical neutralization of the stable P(OK)(3)(.)t and HPO(OH)2(.+) ions forms stable molecules that are detected as survivor ions following reionization. Collisional activation of neutral trihydroxyphosphine, P(OH)(3), results in unimolecular isomerization to the more stable phosphorous acid, HPO(OH)(2). Ab initio calculations, carried out at the MP4(SDQ)/6-31+G(d)/MP2(FULL)/6-3+G(d) level of theory predict all the species under study to be stable equilibrium structures. The proton affinity of phosphoric acid is calculated as 807 kJ mol(-1). Vertical neutralization of P(OH)(4)(+) deposits 129 kJ mol(-1) in the radical formed making it kinetically unstable with respect to hydrogen loss that requires 90 kJ mol(-1). The most stable P(OH)(3) isomer (C-1) is 45 kJ mol(-1) less stable than HPO(OH)(2) but is separated by an isomerization barrier of 224 kJ mol(-1). The P(OH)(3)(.+) cation radical of C-3 symmetry is 136 kJ mol(-1) more stable than HPO(OH)(2)(.+). Vertical neutralization of P(OH)(3)(.+) forms a vibrationally excited neutral 103 kJ mol(-1) above the C-1 isomer, while neutralization of HPO(OK)(2)(.+) results in a 98 kJ mol(-1) excitation in the neutral phosphorous acid. The important role of Franck-Condon effects in the dissociations and isomerizations of gas-phase phosphorus oxoacids and radicals is discussed.