The reaction of α-diketones with malononitrile has been shown to give either simple Knoevenagel adducts or substituted γ-lactams, depending on the proportions of the reactants used. The behaviour of the adduct (la) towards nucleophilic attack has been investigated and the resulting reactions rationalized. The published structures of the products from the reactions of malononitrile dimer with benzil and from malononitrile with benzoin have been reassigned.