The electronic structure of barbituric acid has been investigated, keeping in mind the possibility of tautomerism. It has been found that the triketo form is the most stable, followed by the 4-hydroxy tautomer. The difference in their stabilities decreases on substitution at C₅. Substituents that allow a greater degree of delocalization with the ring system (nitro, bromo, thiol) stabilize the 4-hydroxy tautomer to a greater extent since it is planar. The AM1 method is found to be the best suited for studying the electronic structure of barbituric acid, as it gives the best agreement with the experimental geometries. Of the other two methods investigated, MNDO gives erroneous results for relative energies, while PM3 gives unsatisfactory geometries. The stabilities of the charged species resulting from deprotonation, and the radicals resulting from the removal of a hydrogen atom by heat treatment or irradiation with gamma rays, have also been investigated. Their electronic structures are also discussed.