For the role of monomeric metaphosphate and the nature of the transition states in the alcoholysis of phosphoric monoesters to be examined, phenyl [(R)-16O, 17O, 18O]phosphate and 2,4-dinitrophenyl [(R)- 16O, 17O, 18O]phosphate have been synthesized and the stereochemical course of the methanolysis of phenyl phosphate monoanion and of the dinitrophenyl phosphate dianion has been evaluated. [(R)- 16O, 17O, 18O]Phosphocreatine has also been synthesized and the streochemical course of the methanolysis of this molecule determined. In each case, complete inversion of configuration at phosphorus is observed. It is clear that metaphosphate, if it exists as a true intermediate in these reactions in protic solvent, does not leave the solvent cage in which it is generated. Indeed, product formation occurs more rapidly than rotation of the putative metaphosphate intermediate. These displacements must therefore occur by preassociative mechanisms in which there may be some assistance from the incoming nucleophile. The present results do not allow a distinction to be made between a “preassociative concerted” path (that is, an SN2-like displacement via a very loose transition state) and a “preassociative stepwise” path via a metaphosphate intermediate of very short lifetime.