The presence of different anionic species in natural waters can significantly alter the degradation rates of chlorinated methanes and other organic compounds. Favorable reaction energetics is a necessary feature of these nucleophilic substitution reactions that can result in the degradation of the chlorinated methanes; In this study, ab initio electronic structure theory is used to evaluate the free energies of reaction of a series of monovalent anionic species (OH-, SH-, NO3-, HCO3-, HSO3-, H2PO4-, and F-) that can occur in natural waters with the chlorinated methanes, CCl4, CCl3H, CCl2H2 and CClH3. The results of this investigation show that nucleophilic substitution reactions of OH-, SH-, HCO3-, and F- are significantly exothermic for chlorine displacement, NO3-reactions are slightly exothermic to thermoneutral, HSO3-reactions are slightly endothermic to thermoneutral and HSO4-, and H2PO4- reactions are significantly endothermic. In the case of OH-, SH-, and F- where there are limited experimental data, these results agree well with experiment. The results for HCO3-are potentially important given the near ubiquitous occurrence of carbonate species in natural waters. The calculations reveal that the degree of chlorination, with the exception of substitution of OH-, does not have a large effect on the Gibbs Tree energies of the substitution reactions. These results demonstrate that ab initio electronic structure methods can be used to calculate the reaction energetics of a potentially large number of organic compounds with other aqueous species in natural waters and can be used to help identify the potentially important environmental degradation reactions.