In this study, we determined the crystal structures of the apo form, binary and ternary complexes of the Arabidopsis alkenal (double bond) reductase, encoded by At5g16970. This protein, one of 11 homologues in A. thaliana, is most closely related to the Pinus taeda phenylpropenal (double bond) reductase, involved in, for example, heartwood formation. Both enzymes also have essential roles in plant defense, and can function by catalyzing the reduction of the 7–8 double bond of phenylpropanal substrates, such as p-coumaryl and coniferyl aldehydes in vitro; At5g16970 is also capable of reducing toxic substrates with the same alkenal functionality, such as 4-hydroxy-2E-nonenal. The overall fold of At5g16970 is similar to that of the Zn-independent medium-chain dehydrogenase/ reductase superfamily, the members of which have two domains and are dimeric in nature, i.e. in contrast to their original classification as being Zn-containing oxidoreductases. As provisionally anticipated from the kinetic data, the shape of the binding pocket can readily accommodate p-coumaryl aldehyde, coniferyl aldehyde, 4-hydroxy-2E-nonenal and 2-alkenals. However, the enzyme kinetic data among these potential substrates differ, favoring p-coumaryl aldehyde. Tyr260 is proposed to function as a general acid/base for hydride transfer. A catalytic mechanism for this reduction, and its applicability to related important detoxification mammalian proteins, is proposed.