The synthesis of sym-monothiopyrophosphate (MTP) is described and the relative hydrolysis rate constants are reported for the dianion, trianion, and tetraanion in aqueous solution. The MTP-dianion undergoes hydrolysis at a rate 5 x ro7-fold faster than the dianion of pyrophosphate (PP,) and the trianion undergoes hydrolysis 2.7 x 107-fold faster than PP,-trianion. The entropy of activation for the hydrolysis of MTP-trianion is 0.2 calldeglmol, and the enthalpy of activation is 19.7 kcallmol. The hydrolysis appears to involve a dissociative transition state. The hydrolysis of the tetramethyl ester of MTP, in contrast, appears to involve an associative transition state. The Mg complex of MTP is a reusonable substrate for PP,-dependent phosphofmctokinase and inorganic pyrophosphatase, despite its larger size relative to PP,. Therefore, MTP is tolerated within the active sites and i n the transition states of enzymes that catalyze phosphoryl transfer from PP, to acceptor molecules. These observations are consistent with the transition states for these enzymes being dissociative in nature, although they do not prove that they are dissociative. MTP is, on the other hand, not a substrate for acetyl-CoA synthase, an enzyme that utilizes the Mg complex o f PP, as a specific acceptor for the adenylyl group, a n alkylphosphoryl group. Inasmuch as nonenzymatic reactions of this type involve associative transition states, it may be that the larger size of MTP relative to PP, undermines the catalytic process in the active site of *To whom correspondence should be addressed.