Electronic structure methods were used to calculate the aqueous reaction energies for hydrogenolysis, dehydrochlorination, and nucleophilic substitution by OH- of 4,4‘-DDT. Thermochemical properties ΔHf° (298.15 K), S° (298.15 K, 1 bar), ΔGS (298.15 K, 1 bar) were calculated by using ab initio electronic structure calculations, isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for a series of DDT type structures (p−C6H4Cl)2−CH−CCl3, (p−C6H4Cl)2−CH−CCl2•, (p−C6H4Cl)2−CH−CHCl2, (p−C6H4Cl)2−CCCl2, (p−C6H4Cl)2−CH−CCl2OH, (p−C6H4Cl)2−CH−CCl(O), and (p−C6H4Cl)2−CH−COOH. On the basis of these thermochemical estimates, the overall aqueous reaction energetics of hydrogenolysis, dehydrochlorination, and hydrolysis of 4,4‘-DDT were estimated. The results of this investigation showed that the dehydrochlorination and hydrolysis reactions have strongly favorable thermodynamics in the standard state, as well as under a wide range of pH conditions. For hydrogenolysis with the reductant aqueous Fe(II), the thermodynamics are strongly dependent on pH, and the stability region of the (p−C6H4Cl)2−CH−CCl2•(aq) species is a key to controlling the reactivity in hydrogenolysis. These results illustrate the use of ab initio electronic structure methods to identify the potentially important environmental degradation reactions by calculation of the reaction energetics of a potentially large number of organic compounds with aqueous species in natural waters.