The effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation energies of P-.(CH3)(3)X (X = CH3, SCH3, OCH3, OH, CN, CF3, Ph) were studied via high-level ab initio molecular orbital calculations. Two alternative definitions for the radical stabilization energy (RSE) were considered: the standard RSE, in which radical stability is measured relative to H-P(CH3)3X, and a new definition, the alpha-RSE, which measures stability relative to P(CH3)(2)X, We show that these alternative definitions yield almost diametrically opposed trends; we argue that alpha-RSE provides a reasonable qualitative measure of relative radical stability, while the standard RSE qualitatively reflects the relative strength of the P-H bonds in the corresponding H-P(CH3)(3)X phosphines. The P-.(CH3)(3)X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3p(sigma)-type orbital (that occupies the position of the "fifth ligand"), and the sigma* orbitals of the axial bonds. Consistent with this picture, the radical is stabilized by resonance (along the axial bonds) with configurations such as X-P.+(CH3)(3) and X-. P(CH3)(3). As a result, substituents that are strong sigma-acceptors (such as F, OH, or OCH3) or have weak P-X bonds (such as SCH3) stabilize these configurations, resulting in the largest apicophilicities and alpha-RSEs. Unsaturated pi-acceptor substituents (such as phenyl or CN) are weakly stabilizing and interact with the 3p(sigma)-type orbital via a through-space effect. As part of this work, we challenge the notion that phosphorus-centered radicals are more stable than carbon-centered radicals.