High-level ab initio quantum chemical methods have been used to calculate the radical stabilization energies (RSEs) of phosphonyl radicals XYP(O)center dot bearing a range of substituents X and Y. The main influences on these radicals' stabilities are sigma-effects. Due to the high positive charge on phosphorus, sigma-withdrawal is destabilizing, and sigma-donation is stabilizing. The pyramidal geometry at phosphorus minimizes the effect of stabilization by pi-delocalization, while the potentially stabilizing effect of lone-pair donation is outweighed by concomitant sigma-withdrawal. Thus, the calculated RSEs of phosphonyl radicals XHP(O)center dot increase in the order X = F < Me3N+ < MeO < CF3 < Bu-t < Me2N < NC < H < Ph < MeS < Me3Si. The tautomeric hydroxyphosphinyl radicals X(OH)P center dot exhibit a different set of substituent effects, with RSEs increasing in the order X = CF3 < Me2N < Me3N+ < MeO < Bu-t < H < MeS < Me3Si < F < NC < Ph. In these radicals, both the sigma- and pi-properties of the X substituent influence stability, in tandem with those of the OH group. A comparison of the absolute enthalpies of isomeric phosphonyl and hydroxyphosphinyl radicals indicates that the hydroxyphosphinyl radicals X(OH)P center dot are more stable than the phosphonyl radicals XYP(O)center dot. This is not a common situation in phosphorus chemistry. It is primarily attributed to the greater phosphorus p character of the singly occupied molecular orbital (SOMO) in the hydroxyphosphinyl radicals compared with the phosphonyl tautomers. As in closed-shell phosphorus species, the magnitude of the effect is modulated by the electronegativity of the substituent X.