2-Bromo-1-phenylethylidenemalonic ester reacts with 1-benzyl-1,4-dihydronicotinamide (BNAH) in acetonitrile under illumination at λ > 320 nm to give three debrominated products: an α,β-unsaturated ester, a β,γ-unsaturated ester and a dimer. In the presence of magnesium ion, the β,γ-unsaturated ester is formed exclusively. Using deuterated BNAH-4,4-d 2 as reagent and spin-trapping technique for monitoring reactive intermediate, it has been shown that the reaction proceeds via two different mechanistic pathways: e - transfer-debromination-H abstraction or dimerization for the formation of the α,β-unsaturated ester and the dimer, and e - transfer-debromination-e - transfer-H + transfer for the formation of the β,γ-unsaturated ester.