Time-resolved Fourier transform infrared absorption spectroscopy measurements and B3LYP/cc-pVDZ calculations have been conducted to characterize the reaction dynamics of a remarkable photoinduced 1,3-Cl sigmatropic rearrangement reaction upon 193 or 266 nm excitation of the model systems acryloyl chloride (CH2CHCOCl) and crotonyl chloride (CH3CHCHCOCl) ill Solution The reaction is elucidated to follow nonadiabatic pathways via two rapid ISC processes, S-1 -> T-1 and T-1 -> S-0, and the S-1/T-1 and T-1/S-0 surface intersections are found to play significant roles leading to the nonadiabatic pathways The S-1 -> T-1 -> S-0 reaction pathway involving the key participation of the T-1 state is the most favorable, corresponding to the lowest energy path. It is also suggested that the photoinduced 1,3-Cl migration reaction of RCHCHCOCl (R = H, CH3) proceeds through a stepwise mechanism involving radical dissociation-recombination, which is quite different from the generally assumed one-step concerted process for pericyclic reactions.