Reduction of various α,β-unsaturated ketones (3a–g) and (4a–d) in methanol by the benzothiazoline (1) in the presence of aluminium chloride gives, in all cases, the corresponding saturated ketones (5a–g) and (6a–d) without any of the unsaturated or saturated alcohol. Reduction of α,β-unsaturated esters (7a,b) similarly gives the saturated esters (9a,b), while reaction of cinnamaldehyde (8) with compound (1) does not occur at all. Among the Lewis acids examined, aluminium chloride gives the best results. Reduction of 2′-azachalcone (21) with 2-phenyl[2-²H]benzothiazoline reveals that, in the reduction product, the deuterium atom is located at the β-position with respect to the carbonyl group. The result obtained from the reduction of the same substrate with compound (1) in methan[²H]ol shows that no incorporation of a hydrogen atom from the solvent takes place and suggests (indirectly) that the introduced hydrogen atom at the α-position of the product comes from the benzothiazoline (1). The reaction of (Z)-1,2-dibenzoyl-1,2-diphenylethylene (30) with compound (1) in the presence of aluminium chloride stereospecifically yields meso-1,2-dibenzoyl-1,2-diphenylethane (31). This shows that the transfer of two hydrogens from compound (1) to the carbon–carbon double bond of the enone proceeds via cis-addition. Experiments with ethyl phenylpropiolate (28) also support cis-reduction for the present conjugate reduction. These results are interpreted in terms of a mechanism involving synchronous transport of a pair of hydrogens from the benzothiazoline (1); i.e., a cyclic addition of the two hydrogens either in exact or nearly exact concurrence.