Since N,N-dibenzyl-1-cyclohexenecarbothioamide is photo-isomerized to the optically active beta-thiolactam with the retention of the single-crystal form, the mechanism of chirality induction was identified by X-ray crystal structure analyses during the process of the reaction [Hosoya et al. (2002). Bull. Chem. Soc. Jpn, 75, 2147-2151]. In order to clarify the mechanism of hydrogen transfer in the reaction, the H atoms of the benzyl groups were replaced with deuterium atoms. The crystal structure after photoisomerization was analyzed by neutron diffraction. One of four deuterium atoms of the two benzyl groups is transferred to the C atom of the cyclohexene ring. The absolute configuration of the -C*HD- group (chiral methylene) in the photoproduct beta-thiolactam revealed that the deuterium atom occupies the equatorial position. This suggests that the deuterium atom is not transferred from the benzyl group of a neighbouring molecule, but from one of the benzyl groups within the molecule.