The Br3P-Br- and I3P-I- bond strengths have been determined by measuring thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. The results are combined with previously determined values for the PF4- and PCl4- systems to determine the effect of the terminal atom on hypervalent bond strengths. The bond strengths correlate very well with the difference in the electronegativities of phosphorus and the halogen atoms. Computational results at the B3LYP/6-311+G(d) level indicate that PCl4- and PBr4- have seesaw geometries and are good examples of three-center, four-electron bonding. PF4- has a,similar geometry, but the bonding is more ionic, whereas PI4- is tetrahedral, consistent with greater steric crowding.