Four reactions of potential importance in the catalytic recombination of H + OH in the presence of phosphorus compounds have been studied by ab initio quantum chemical and RRKM methods. These are the reactions HOPO + OH --> (HO)(2)PO --> H2O + PO2 (3), HOPO + H --> P(OH)(2) --> H2O + PO (4), (HO)(2)PO + H --> P(OH)(3) --> H2O + HOPO (5), and HOPO2 + H --> (HO)(2)PO --> H2O + PO2 (6). Each of these reactions takes place by recombination with small (<5 kcal mol(-1)) or no barrier to form an adduct. The subsequent decomposition of the adducts occurs by the elimination of H2O through four-centered transition states. The thermochemistry of these reactions including the heats of formation of each of the adducts and the appropriate transition states was computed by the Gaussian G3X and G3X2 methods. Overall rate coefficients for each of these reactions at temperatures from 1000 to 2000 K and at pressures between 1 and 10 000 Torr were computed by the MultiWell code developed by Barker (Barker, J. R. Int. J. Chem. Kinet. 2001, 33, 232). For each reaction, there is very little stabilization of the adduct; hence the reactions are essentially pressure-independent. For reaction 6, its overall rate coefficient for the addition/decomposition mechanism greatly exceeds that previously derived for an abstraction transition state (Haworth, N. L.; Bacskay, G. B.; Mackie, J. C. J. Phys. Chem. A 2002, 106, 1533-1541).