The crystal and molecular structure of 1,3-dimethyl-8-azaxanthine (HDAX) monohydrate has been determined by X-ray diffraction. The compound exhibits the N8-H tautomer in the solid state and hydrogen-bonded dimers are formed. Molecular orbital calculations have been performed for this compound, its analogues 1,3-dimethyl-2-thio-8-azaxanthine, 3-methyl-8-azaxanthine and 3-methyl-2-thio-8-azaxanthine and their anionic forms with the MOPAC program. The most stable tautomer is shown to be N7-H for the neutral compounds. Calculations have also been performed for the hydrogen-bonded dimer (HDAX·H₂O)₂, a stabilization energy of -19.76 kcal mol^-1 having been found, which may explain why the N8-H tautomer is found in the solid state. The role of the molecular orbitals in possible coordination to metal ions is discussed for all the species.