Axial- and equatorial-substituted pentacoordinate oxyphosphoranes, P(OH)(4)X, and oxyphosphorane monoanions, PO-(OH)(3)X, where X = OH, H, F, CH3, and NH2, have been studied using ab initio quantum chemical methods, leading to complete optimized structures, total energies, and harmonic vibrational frequencies for the most stable conformer of each compound. Thermodynamic results including gas-phase apicophilicities for each substituent as a function of protonation state along with estimates of the gas-phase proton affinity for each substituted oxyphosphorane monoanion are also presented. Split-valence basis sets ranging in quality from 3-21+G(d) to 6-311+G(d,p) are employed, and the effects of electron correlation on the structure and thermodynamics are estimated through the use of Moller-Plesset perturbation theory [MP2-MP4(SDTQ)]. The effect of aqueous solvation on the gas-phase apicophilicities is also estimated through the use of a continuum reaction field model. Final predictions for the gas-phase apicophilicities (kJ mol(-1)) determined at the MP4(SDTQ) 6-31+G(d)/MP2 6-31+G(d)+ZPVE level are F(+25.5) > OH(0.0) > H(-5.7) > CH3(-17.7) > NH2(-28.0) for the neutral oxyphosphoranes and F(+30.2) > H(+1.6) approximate to [CH3] approximate to OH(0.0) > NH2(-18.7) for the oxyphosphorane monoanions. Certain substituted oxyphosphoranes, including X = Cl and SH, are found to be unstable with respect to dissociation and thus are not considered. The effect of aqueous solvation, as estimated by a continuum reaction field, is modest and does not change the overall apicophilicity ordering significantly in either the neutral or anionic systems. Comparison is made with the corresponding hydrogen phosphoranes as well as the empirically based model of Holmes.