Crystals of 4,2'-anhydro-5-(beta-D-arabinofuranosyl)uracil, (I), obtained from an aqueous solution, were characterized as the dihydrate, C(9)H(10)N(2)O(5).2H(2)O, (Ia). In air, these crystals slowly transform to the monohydrate, C(9)H(10)N(2)O(5).H(2)O, (Ib), but remain crystalline. The solid-state transformation proceeds with the loss of one water molecule and a rearrangement of hydrogen-bonded layers of molecules. The furanose ring in (I) has an approximate C4'-exo,O4'-endo twist conformation. The central five-membered ring is slightly puckered. The uracil group is planar within experimental uncertainty.