Rates and energy barriers of degenerate halide substitution on tetracoordinate halophosphornum cations have been measured by NMR techniques (VT and EXSY) using a novel experimental design whereby a chiral substituent (Bu-s) lifts the degeneracy of the resultant salts. Concomitantly, a viable computational approach to the system was developed to gain mechanistic insights into the structure and relative stabilities of the species involved. Both approaches strongly suggest a two-step mechanism of formation of a pentacoordinate dihalophosphorane via backside attack followed by dissociation, resulting in inversion of configuration at phosphorus. The experimentally determined barriers range from <9 kcal mol(-1) to nearly 20 kcal mol(-1), ruling out a mechansm via Berry pseudorotation involving equatorial halides. In all cases studied, epimerization of chlorophosphonium chlorides has a lower energy barrier (by 2 kcal mol(-1)) than the analogous bromo salts. Calculations determined that this was due to the easier accessibility in solution of pentacoordinate dichlorophosphoranes when compared to analogous dibromophosphoranes. In line with the proposed associative mechanism, bulky substituents slow the reaction in the order Me < Et < Pr-i < Bu-t. Bulky substituents affect the shape of the reaction energy profile so that the pentacoordinate intermediate is destabilized eventually becoming a transition state. The magnitude of the steric effects is comparable to that of the same substituents on substitution at primary alkyl halides, which can be rationalized by the relatively longer P C bonds. The reaction displays first-order kinetics due to the prevalence of tight- or solvent-separated ion pairs in solution. Three-dimensional reaction potential energy profiles (More O'Ferrall-Jencks plots) indicated a relatively shallow potential well corresponding to the trigonal bipyramid intermediate flanked by two transition states