The charge-separation process of the C2H4 + Cl-2 reaction in water has been studied with ab initio molecular orbital methods using a cluster model composed Of C2H4 + Cl-2 + 4H(2)O. The final charge-separated complex was calculated to lie 4.3 kcal mol(-1) lower in energy than the C2H4 + Cl-2 + 4H(2)O asymptote and 0.6 kcal mol(-1) higher than the complex produced in the reactant region at the MP4(SDQ,full)/aug-cc-pVDZ level of theory with the zero-point energy correction and with the basis-set superposition error correction. The total free energy of the final charge-separated complex relative to the reactant complex in water at 298.15 K and I atm was estimated to be -5.2 kcal mol(-1) using the gas-phase thermodynamic quantities combined with the solvation free energies obtained with the polarized continuum model. This result strongly suggests that the charge-separation process in water at the normal temperature and pressure is spontaneous.