The preparation and crystal and molecular structure of the osmium tetraoxide bispyridine ester of 1-methylthymine are reported. The complex crystallizes in the triclinic system, space group P1, with a = 11.493(6)A, b = 16.655(7)A, c = 6.082(2)A, alpha = 92.07(3) degrees, beta = 90.58(3) degrees, gamma = 71.36(4) degrees, V = 1102.4 A3, Dm = 1.85(1) g cm-3, DC = 1.84 g cm-3. The unit cell contains 2 osmium tetraoxide bispyridine esters of 1-methylthymine, 2 waters of crystallization and 1 disordered pyridine of solvation. Intensities for 3814 independent reflections were collected by counter methods. The structure was solved by standard heavy-atom techniques and has been refined by full-matrix least squares, based on F, to a final R value of 0.065. The osmium complex binds as a cis osmate ester to the C(5)-C(6) bond of the methylated pyrimidine in a fashion which is expected to be similar to the binding of the complex to thymidine residues in nucleic acids. The conformation of the 1-methylthymine ester is that of a half chair with C(6) showing a substantial deviation, 0.55 A, from the best mean plane of the thymine moiety. The primary coordination sphere about the Os(VI) atom is completed by 2 axial Os=O bonds and the binding of the 2 pyridine ligands in cis positions in the equatorial plane containing the ester linkages. The O=Os=O group is substantially nonlinear, 164.0(5) degrees, and this nonlinearity is attributed to intracomplex electronic effects.