Diphenylphosphinoyl (−P(O)Ph2) and diphenylthiophosphinoyl (−P(S)Ph2) respectively are modest and strong β-effect functionalities. When these groups are antiperiplanar to mesylate, the substrates solvolyze through unimolecular ionization. In contrast, substrates with the synclinal (gauche) arrangement react bimolecularly with solvent. The anti/gauche rate ratio is 440 for the oxide and 3.2 × 106 for the sulfide at 25 °C. The antiperiplanar sulfide in fact reacts 220 times more rapidly than the analogous cyclohexyl substrate at 25 °C, despite the strong electron-withdrawing nature of diphenylthiophosphinoyl. The α-secondary hydrogen/deuterium kinetic isotope effects of 1.21 for the anti oxide and 1.26 for the anti sulfide suggest high sp2 character in the transition state, as expected for stabilization by hyperconjugation in a vertical mechanism. Energies calculated at the MP2 level show that the geometry with the P−C bond parallel to the empty carbocation p orbital is more stable than the perpendicular geometry by 22.94 kcal mol-1 for PH2, 8.80 kcal mol-1 for P(O)H2, and 10.54 kcal mol-1 for P(S)H2, confirming significant hyperconjugation for these substituents. The global minimum, however, is the bridged structure (four-membered rings for the oxide and sulfide, three-membered ring for the simple phosphine), so that the mechanistic choice between a vertical mechanism (hyperconjugation) and a nonvertical mechanism (bridging) is not clear-cut.