The structure and conformation of 2′,3′-dideoxy-2′,3′-didehydrocytidine (2′,3′-dideoxycytidin-2′-ene, d4C), a potent inhibitor of the human immunodeficiency virus, was determined by X-ray crystallography. The nucleoside crystallizes in the orthorhombic space group P212121with cell dimensions a = 8.603(1),b= 9.038(1),c= 25.831(2) A and with two independent molecules in the asymmetric unit (Z= 8). Atomic parameters were refined by full-matrix least squares to a final value of R = 0.033 for 2258 observed reflections. The molecules are quite flexible: in molecule A the glycosyl torsion angle (XCN) is 61.3° and the -CH2OH side chain is in thegauche+orientation while in molecule B XCN= 19.8° and the side chain istrans. The five-membered rings are slightly puckered (∼0.1 Å), 04′ beingendoin molecule A andexoin molecule B. A mechanism is proposed for the known instability of 2′,3′-unsaturated nucleosides.