Methyl mercury, an important pollutant, decomposes photolytically when exposed to sunlight. Methyl mercury chloride has been shown to yield CH3 and HgCl radicals upon photodecomposition under UV irradiation. We have calculated spectral transition energies for a number of methylmercury species using quantum mechanical methods, specifically the Hartree-Fock method, the Moller-Plessett second order perturbation theory method (MP2), and the configuration interaction singles method using polarized double-zeta relativistic effective core potential basis sets. We find that singlet to tripler absorptions occur at lower energy than that of singlet to singlet absorptions, by about 2-3 eV. The calculated singlet to tripler (S-T) energy is much lower for 1-coordinate CH3Hg+ than for the 2-coordinate species CH3HgL, where L=CH3-, OH2, OH-, Cl- or SH-. Of the 2-coordinate species studied CH3HgOH2+ and CH3HgSH show the lowest energy S-I : transitions, with calculated maxima just below 5.0 eV (at the MP2 level). They should, therefore, show significant absorption at 4.4 eV, the cutoff in the solar spectrum produced by the ozone layer. The lowest energy triplet states of these compounds are calculated to be dissociative, e.g., CH3HgCl decomposes to a CH3 radical and the neutral HgCl radical, while CH3HgOH2+ decomposes to CH3 and HgOH2+. The dissociation of the CH3 group can be understood by considering the compositions of the highest energy occupied and lowest energy unoccupied molecular orbitals (HOMO＇s and LUMO＇s) of the singlet states, which are Hg-C sigma-bonding and Hg-C sigma-antibonding, respectively. Creation of the triplet state depopulates the HOMO and populates the LUMO, greatly reducing the Hg-C bond strength. Reaction energies have also been calculated for the formation of various CH3HgL species from CH3Hg+ and the ligands, L. The reaction energetics indicate that CH3HgOH2+ and CH3HgOH are the most important species even in the presence of appreciable Cl- or SH-. Therefore, the methyl Hg species decomposed by sunlight in natural water systems is probably CH3HgOH2+.