The X-ray crystal structure of 2-chloro-2 -deoxyadenosine was determined at low temperature [space group orthorhombic P2 1 2 1 2, a = 7.493(6), b = 16.894(4), c = 9.065(4) 9, V, = 1147.6(11) 9 3 , 4097 unique reflections, R = 0.050]. The ribose unit is in 2 3 T conformation, and the orientation around the C1 -N glycosylic bond is syn with respect to the base. The 2-chloro-substitution leads to small but significant changes of the covalent geometry of the base at the substitution side, particularly to widening of the endocyclic angle at C2. The crystal packing is unusual foradenine nucleosides: molecules form stacks with parallel bases which are aligned by chains of short chlorine-chlorine contacts.