The hydrolysis of H-phosphonodiesters bearing a vicinal hydroxyl group is found to be subject to two competing reaction pathways in aprotic organic media. An observation of the increased proportion of cis-1,2-diol leaving with decrease of the water content is interpreted in terms of a change of the hydrolytic mechanism on changing the reaction medium from aqueous to nonaqueous. The hydroxyl group in cis-1,2-diol monoanions hydrogen bonds strongly to the adjacent oxyanion, implying a low-energy route closely related to reactions, catalyzed by large ribozymes.