The 1:1 molecular complex of 9-methyladenine and 2-thiohydantoin (C6H7N 5.C3H4N2OS) is monoclinic, space group C2/c with unit-cell parameters a = 21.140(3), b = 7.085(1), c = 16.101A and fl = 103.27 (2) °, with eight complexes per unit cell (D c 1.499, D o = 1.508 Mg m-a). The intensity data were collected on an automated diffractometer using Nbfiltered Mo K~ radiation, and the structure was solved using direct methods. The structure was refined in two blocks (non-hydrogen atoms anisotropic, hydrogens isotropic); the refnement converged to a final conventional R (f) value of 3.5 % (4.9 % weighted) for the 2475 significant data. The structure consists of ribbons with alternating adenine and hydantoin moieties linked via (hydantoin)N-H...N(adenine) hydrogen bonds. The ribbons are held together via N-H... N hydrogen bonds involving the 9-methyladenine molecules. The sulfur and oxygen atoms of the 2-thiohydantoin molecules do not appear to act as acceptors in strong hydrogen bonds.