Publisher Summary The epoxidation of styrene and the formation of phenylacetaldehyde using Mn-porphyrin catalysts were analyzed in this chapter with QM/MM ONIOM calculations. For the epoxidation reaction, the calculations suggest a stepwise mechanism, in which a radical intermediate is formed before the concerted intermediate. The calculations reveal that the oxidation of the porphyrin with iodosylbenzene proceeds without a reaction barrier. The lowest energy barrier for the formation of the radical intermediate is 51 kJ/mol in the triplet state. The lowest energy barrier for the formation of the concerted intermediate from the radical intermediate is 15 kJ/mol in the quintet state. The barrier to form the side product, phenylacetaldehyde, is predicted to be much higher in energy, consistent with the selectivity of this catalyst to form styrene oxide over phenylacetaldehyde. The polarizable continuum model was used to take the solvent dichloromethane into account. In the presence of solvent, the energy barriers did not change significantly for any of the steps in the mechanism compared to gas-phase calculations.