The phosphor group of 2-phosphopropane-1,2-diol migrates to the 1-position on heating in aqueous acid. This migration occurs by two routes. The direct route is believed to proceed via a pentacoordinate intermediate that must, by Westheimer’s rules, pseudorotate at least once to yield product. In the phospho diester route, the cylic 1,2-phospho diester is formed as an intermediate. Four experiments have been performed to determine the rate constants for each route. These experiments involve the measurements of (a) the overall equilibrium constant, (b) the partition ratio (to the 1- or 2-phospho monoesters) for the hydrolysis of the cyclic diester intermediate, (c) the overall rate of isomerization, and (d) the rate of solvent 18O incorporation into an equilibrium mixture of 1- and 2-phospho compounds. By using chiral 2-[(R)-16O,17O,18O]phospho-(S)-propane-1,2-diol as substrate and determining the configuration both of the 1-phospho product isomer and of the remaining 2-phospho substrate, the direct route has been shown to proceed with retention of configuration at phosphorus, in accord with the predicted behavior for reaction via a pseudorotating pentacoordinate intermediate.