The crystal structure of thymidine (CIoN2OsH14) has been determined from data collected on a Hilger and Watts linear diffractometer and a Wooster four-circle diffractometer. Thymidine is orthorhombic, space group P2~2121, with the following cell dimensions: a= 4.860_ 0.005, b = 13.91 _+ 0.01 and c= 16.32 +_ 0.01/~. The structure was solved by a Patterson interpretation method and the positional and thermal parameters were refined by the method of least-squares. The carbon, nitrogen and oxygen atom thermal parameters were refined anisotropically. The final R value for the 1230 observed reflexions is 0.066 and the average standard deviations in the bond lengths and angles are about 0.006/~, and 0.4 ° respectively.

The structure of inosine (C10N4OsH12) which crystallizes in the space group P21 with one molecule per asymmetric unit and with unit-cell dimensions: a=4.818+0.005, b= 10-45+0.01, c= 10.97+0.01/l, and ,8= 90043 ' + 2", has been determined from X-ray intensity data collected from linear and four-circle diffractometers. The structure was solved by a Patterson function interpretation method and the positional and thermal parameters were refined by the method of least squares, using anisotropic thermal parameters for the non-hydrogen atoms. The final R value for the 1298 observed reflexions was 0.046 and the standard deviations in the bond lengths and angles are about 0.004 ~ and 0-3 ° respectively. The purine ring in inosine is planar, but both O(10) and C(I') are significantly displaced from this plane. The dihedral angle between the base and sugar planes is 71.0 ° and the glycosidic torsion angle, ~0CN is

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