Reaction of an aqueous solution of phosphate ion (pH 7.8) with acetic anhydride in a two-phase system gives acetyl phosphate. Reaction of phosphate with methyl chloroformate yields methoxycarbonyl phosphate. Both reagents are useful for in situ regeneration of ATP from ADP in organic synthetic procedures based on enzyme-catalyzed reactions requiring ATP. Acetyl phosphate has been used for this purpose previously; methoxycarbonyl phosphate is a new compound, and its use in ATP regeneration is also new. The characteristics of methoxycarbonyl phosphate which makes it interesting are its ease of preparation, its acceptability as a substrate for both acetate kinase and carbamate kinase, and its high phosphoryl donor potential. It has the additional attractive feature that the product remaining after phosphoryl transfer, methyl carbonate, decomposes spontaneously in solution and forms methanol and carbon dioxide. These products present no difficulties in workup and avoid the problem of product inhibition which is sometimes troublesome in regeneration schemes based on acetyl phosphate or phosphoenolpyruvate. The principal disadvantage of methoxycarbonyl phosphate as a phosphorylating reagent in ATP regeneration, relative to acetyl phosphate, is that it decomposes inconveniently rapidly under the conditions used for enzymatic synthesis (t1/2=0.3 h at 25 °C, pH 7).