Abstract: In this parper, the decarboxylation reaction of acrylic acid in the singlet and triplet excited states were investigated by semi-empirical self-consistent-field molecular orbital method (AM1). Our results support the mechanism of photolysis proposed th Robert. In combintion with experiment, it can be deduced that the first step of photodecarboxylation reaction is in the singlet pathway, the second step in triplet pathway. The reactants, transition states, reaction intermedi-ates and products corresponding to the singlet and triplet pathways were optimized by gradient techniques. All of the transition states and intermediates were confirmed by full vibrational analysis.

Key words: Acrylic acid, Photodecarboxylation reaction, Semi-empirical self-consistent-field molecular ordital method