High-level ab initio quantum chemical methods have been used to calculate the radical stabilization energies (RSEs) of phosphonyl radicals XYP(=O)· bearing a range of substituents X and Y. The main influences on these radicals' stabilities are σ-effects. Due to the high positive charge on phosphorus, σ-withdrawal is destabilizing, and σ-donation is stabilizing. The pyramidal geometry at phosphorus minimizes the effect of stabilization by π-delocalization, while the potentially stabilizing effect of lone-pair donation is outweighed by concomitant σ-withdrawal. Thus, the calculated RSEs of phosphonyl radicals XHP(=O)· increase in the order X = F < Me3N+ < MeO < CF3 < tBu < Me2N < NC < H < Ph < MeS < Me3Si. The tautomeric hydroxyphosphinyl radicals X(OH)P· exhibit a different set of substituent effects, with RSEs increasing in the order X = CF3 < Me2N < Me3N+ < MeO < tBu < H < MeS < Me3Si < F < NC < Ph. In these radicals, both the σ- and π-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· are more stable than the phosphonyl radicals XYP(=O)·. 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.