An in depth study of the mechanism of the beta-(phosphatoxy)alkyl radical migration is presented. Examples are presented which define the scope and limitations of the migration and show that, in certain cases, it is highly stereoselective. It is shown that phosphoranyl radicals are not intermediates in this rearrangement. A series of experiments with stereochemically-, O-18-, and deuterium-labeled probes indicate that the migration is intramolecular, proceeds through competing 1,2- and 2,3-pathways, and does not involve fragmentation to a cage pair followed by recombination. The deuterium-labeled probe is also applied to the beta-(acyloxy)alkyl migration with the same result. The changing proportions of 1,2- and 2,3-shifts in going from the beta-(phosphatoxy)alkyl to the beta-(acyloxy)alkyl migration are discussed in terms of the conformational equilibria of the two different esters and the Curtin-Hammett principle.