Crystallization of 1-deaza-2'-deoxyadenosine (c 1 A d , 1) from propanol-2 gives two forms of crystals: type A formed firstly as plates, then, on the surface of the plates, type B appeared as needles. Single crystal X-ray analyses shows that the crystals A and B differ mainly in the sugar ring conformation: A adopts the S-type (P = 179.8 o ; C-2'-endo-C-3'-exo; 2 T 3 ) conformation associated with an high-anti base orientation (χ = -90.7 o ) and the γ = 152.3 o across the exocyclic C(4')-C(5') bond; B shows N-type (P = 21.2; C-3'-endo; 3 E) conformation accompanied by a somewhat different anti base orientation (χ = -116.5 o ) an eclipsed orientation of the exocyclic C(4')-C(5') bond (γ = 84.5 o ). No intramolecular hydrogen bonds in both types of crystals can be detected. Unlike c 1 A d , the ribonucleoside (c 1 A, 2) is in the syn conformation (χ = 56.1 o ) in the solid state which is caused by an intramolecular (5')CH 2 OH...N(3) hydrogen bond. The compounds 1 and 2 display very similar conformation in D 2 O solution with a strong preference for the S-type conformation of the furanose ring accompanied by the intramolecular (5')CH 2 OH...N(3) hydrogen bond.