Resonance Raman spectra (RRs) and quantum chemical calculations were used to investigate the photodissociation dynamics of diuron in S2 state. The RRs indicate that the photorelaxation dynamics for the S0 → S2 excited state is predominantly along nine motions: the ring C = C stretch vibration ν12 (1593 cm−1), Ph–N–H wag ν14 (1517 cm−1), CO–N(CH3)2 stretch ν23 (1365 cm−1), CCH wag in plane/ring C = C stretch ν24 (1297 cm−1), ring CH rock in plane/ring deformation ν27 (1233 cm−1), CCH wag in plane ν29 (1151 cm−1), Ph–Cl (para) stretch ν35 (1028 cm−1), Ph–N–H wag ν37 (913 cm−1) and ring breath ν44 (685 cm−1). Dissociation by Ph–Cl (para) cleavage at S2 state directly or relaxation to T2 state by internal conversion (S2 → S1) and intersystem crossing (S1/T2) is expected by ~250 nm irradiation based on the RRS, complete active space self‐consistent field, configuration interaction singles and time‐dependent density functional theory calculations. Copyright © 2012 John Wiley & Sons, Ltd.