Some reactions of fundamental importance in mercury photosensitization have been studied by high level quantum chemical calculations. The reactions of Hg* (P-3(1)) With H-2, CH4, C3H8, and SiH4 go by initial formation of exciplexes, followed by insertion of Hg* into the reactive X-H bond ultimately giving X radicals and H atoms. Intermediates and transition states have been located, notably the bent triplet (3)[H-Hg-H] species. The hydrogen and silane reactions are found to proceed without barriers, whereas the methane and propane reactions require activation energies. In the case of methane the barrier is so high, 12.8 kcal/mol, that the reaction is very inefficient while for propane the barrier is small, 4.9 kcal/mol, leading to the breaking of one of the central C-H bonds. Triplet sensitization of alkenes and the bonding of Hg* with lone-pair ligands are also studied. Very good agreement with the general picture obtained from experiments is obtained. The consideration of both spin-orbit and zero-point vibrational effects are very important for a qualitative understanding of the reactions. Similarities and differences to the corresponding transition metal reactions are discussed.