Ab inirio quantum mechanical methods have been employed to study the mechanisms for the hydrolysis of the pyrophosphate and mu-monothiopyrophosphate (MTP) anions at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of theory using double zeta plus polarization (DZP) and DZP plus diffuse functions (DZP+diff) basis sets. Metaphosphate is found to be a kinetically important species in the hydrolysis of pyrophosphate for three mechanisms examined in this study (S(N)1 dissociation, acid catalysis, and Mg2+ catalysis). The transition state for the unimolecular reaction of HP2O73- trianion to yield a PO3- anion is dissociative, with the reaction coordinate distance as long as 2.91 Angstrom A at the DZP SCF level. This dissociation reaction has a gas phase classical barrier of 25 kcal mol(-1) (DZP MP2) and 26 kcal mol(-1) (DZPfdiff SCF). For the proton-catalyzed hydrolysis of pyrophosphates, the potential energy surface of the pyrophosphate is dissociative if the bridging oxygen atom is protonated. This dissociation thus yields a metaphosphate and an orthophosphoric acid (H3PO4) A hypothesis is formulated to explain the catalytic effect of the Mg2+ cation on the hydrolysis of the pyrophosphate. We predict that one of the P-O bridging bonds is activated in the Mg2+. H2P2O7(2-) complex. Upon hydrolysis, the Mg2+. H2P2O72- complex may isomerize to the H2PO4-. Mg2+. PO3- initially; then the H2PO4-. Mg2+. PO3- complex may be captured by a water molecule to form the H2PO4-. Mg2+. H2PO4- complex. The classical activation barrier for the isomerization of Mg2+. H2P2P72- to H2PO4-. Mg2+. PO3- is 7.8 kcal mol-(1) at the DZP MP2 level. The H2PO4-. Mg2+. PO3- complex lies 8.9 kcal mol(-1) (DZP SCF) lower in energy than the Mg2+. H2P2O72- complex. Therefore, the isomerization of Mg2+. H2P2O72- to H2PO4-. Mg2+. PO3- is both thermochemically and kinetically feasible.