2,2′-Anhydro-3′-O-acetyl-2′-thio-1-β-D-arabinofuranosylcytosine hydrochloride crystallizes in space group P212121 witha=10.373(1),b=14.517(2),c=9.496(1) Å. Reflections were measured on a diffractometer and the structure was solved by direct methods. Least-squares refinement converged atR=0.056 andR w =0.063. The study of this crystal structure showed that the alterations in the geometry of 2,2′-anhydro-1-β-D-arabinofuranosylcytosine when the bridge oxygen is replaced by sulfur are localized in the region of the fused ring. The angle at the bridge atom decreases to nearly 90°, with concomitant enlargements of about 5°–7° in the angles opposite to the bridge atom. Angle C(1′)-N(1)-C(6) decreases by 4°. Also, the amount of double bond character in the bond formed by C(2) and the bridge atom decreases. A survey of the conformational features of S,2′-cyclonucleosides and comparison with O,2′-cyclonucleosidcs showed that their preferred conformations are the same. However, S,2′-cyclonucleosides exhibit a wider range of P and χ values. This correlates with a greater ease of the sulfur containing five-membered ring to pucker as compared to the oxygen-cyclo ring.