Comprehensive theoretical investigations were performed on the electronic singlet and triplet pipi* and npi* transition energies of 2-thiouracil, 4-thiouracil, 2,4-dithiouracil, and their methyl derivatives in the gas phase and in water and acetonitrile solutions. The computed results were compared with the corresponding properties of uracil. The ground state geometries of different tautomers were optimized at the B3LYP level with use of the 6-311++G(d,p) basis set. Single point energy calculations were performed at the MP2/cc-pVTZ level by using the B3LYP/6-311++G(d,p) geometries. Geometry optimizations for the most stable tautomers of each species were also performed at the MP2/cc-pVTZ level. Vertical electronic transition energies were computed at the time-dependent density functional theory level (TD-DFT), using the B3LYP functional and the 6-311++G(d,p) basis set and utilizing the B3LYP/6-311++G(d,p) geometries. The effects of water and acetonitrile solutions on the ground and excited states were computed by using the Polarizable Continuum Model (PCM). Computed electronic transition energies were found to be in good agreement with the corresponding experimental data. It was found that thiouracils would exist mainly in the keto-thione tautomeric form in the gas phase and in solutions. The similarity and differences with uracil are also discussed.