A sophisticated mechanistic simulation model was developed to describe the ultraviolet degradation of H2S in waste gas. The photochemical reactor was simulated with a model that includes 47 chemical and photochemical reactions involving 19 chemical species, material balances for each species in the model as well as the gas flow pattern. Simulation shows that the UV degradation of H2S in waste gas is a highly efficient process. The model was validated by comparing experimental data with model. Although the model overestimates the process efficiency somewhat, comparable trends were found in two different reactors, confirming that the model is sound and can be used for feasibility studies. The model predicts that H2SO4 is the main reaction product at very low H2S concentration, as found experimentally, and that SO2 is predominant at high H2S concentration. Photolysis is a simple and effective way to decompose H2S to less harmful products. In addition, the model was extended to include the photochemical degradation of NO and H2S + NO mixtures. The extended model includes 91 chemical and photochemical reactions and 30 chemical species. The model predicts that the ultraviolet degradation of H2S + NO mixtures is more efficient than the degradation of either of the compounds in isolation. (C) 2014 Elsevier B.V. All rights reserved.