The reasons why colchicine undergoes photochemical cyclization to give lumicolchicine, whereas thiocolchicine does not, are investigated by theoretical methods. The relative stability of the reagent and of the products is computed at DFT level of theory and indicates that such a photochemical reaction is energetically possible. The photochemical reaction path of thiocolchicine is studied within the relaxed energy scan, at MC-SCF level of theory. The results are again rather similar to those of colchicine. Finally, the spin-orbit coupling between S-1 and T-1 has been computed and are two order of magnitude greater than those of colchicine, that makes plausible that spin-orbit is the main reason of the observed reaction quenching. (c) 2008 Elsevier B.V. All rights reserved.