Phosphate hydrolysis plays a major role in many biological processes. To fully understand such processes, it is essential to obtain a quantitative description of the corresponding reactions in solution. Here we present a systematic theoretical study of the nonenzymatic hydrolysis of monomethyl phosphate via the nucleophilic attack at the phosphorus center and explore the energetics of various reaction mechanisms as well as the structures and charge distributions of corresponding reaction intermediates and transition states. We consider all the relevant protonation states of the phosphate oxygens and two forms of the nucleophile that correspond to hydrolysis by OH- and by neutral water. These systems are studied by using ab initio quantum mechanical calculations coupled with the Langevin dipoles (LD) and polarized continuum (PCM) solvation models. The reliability of the calculations is verified by comparing calculated and observed values of key reaction free energies, activation free energies, and kinetic isotope effects. Combining the calculated and observed information provides "consensus" free-energy surfaces for the hydrolysis of phosphate monoesters. It is found that the barriers for the associative pathways are similar to that of the dissociative pathways. Thus an enzyme active site could select either of these mechanisms depending on the particular electrostatic environment.