Various molecular species that can be populated during the photoreaction of carbon tetrahalides CX4 (X = Cl, Br, I) in the gas phase and in solution have been studied by ab initio and density functional theory (DFT) calculations. Geometries, energies, and vibrational frequencies of CX4, CX3, CX2, C2X6, C2X5, C2X4, X-2, and the isomer X2CX-X were calculated and transition states connecting these species were characterized. Spin-orbit DFT (SODFT) computations were also performed to include the relativistic effects, which cannot be neglected for Br and I atoms. The calculated potential energy surfaces satisfactorily describe the reactions of the photoexcited CX4 molecules. In the gas phase, the initial C-X bond rupture in CX4 is followed by secondary C-X breakage in the CX3 radical, leading to CX2 and 2X, and the formation of C2X6 or C2X4 through bimolecular recombination of the CX3 or CX2 radicals is favored thermodynamically. In solution, by contrast, the X2CX-X isomer is formed via X-X binding, and two CX3 radicals recombine nongeminately to form C2X6, which then dissociates into C2X4 and X-2 through C2X5. The Raman intensities and the vibrational frequencies, as well as the absorption spectra and oscillator strengths of the Br2CBr-Br isomer in the gas phase and in various solvents were computed and the calculated absorption and Raman spectra of the Br2CBr-Br isomer in various solutions are in good agreement with the experimental data. The natural population analysis indicates that the Br2CBr-Br isomer corresponds to the recently reported solvent-stabilized solvated ion pair (CBr3+//Br-) solv in the highly polar alcohol solvent. The singlet-triplet energy separations of the CX2 radicals in the gas phase and in solution were evaluated with high level computational methods, and the optimized geometric parameters are in good agreement with the experimental results. The geometric and energetic differences between the singlet and triplet states were explained by the electronic properties of the CX2 radicals. C2X4, C2X5, and C2X6 (X = Br, I) in the gas phase and in solution were optimized at different computational levels, and the optimized geometric parameters of C2I4 are in very good agreement with the experimental data.