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In the present work, mechanism of the O2(1Δg) generation from the reaction of the dissolved Cl2 with H2O2 in basic aqueous solution has been explored by the combined ab initio calculation and nonadiabatic dynamics simulation, together with different solvent models. Three possible pathways have been determined for the O2(1Δg) generation, but two of them are sequentially downhill processes until formation of the OOCl− complex with water, which are of high exothermic character. Once the complex is formed, singlet molecular oxygen is easily generated by its decomposition along the singlet‐state pathway. However, triplet molecular oxygen of O2( Σ3g− ) can be produced with considerable probability through nonadiabatic intersystem crossing in the 1Δg/ Σ3g− intersection region. It has been found that the coupled solvent, heavy‐atom, and nonadiabatic effects have an important influence on the quantum yield of the O2(1Δg) generation. © 2018 Wiley Periodicals, Inc.