The purpose of the present work is to employ theoretical chemical tools to explore the subtle features of the hydrogenation of uracil and its related thioderivatives. The reactions of hydrogenation are followed for the free uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil as well as for adenine-thiouracil base pairs. It is shown for the first time that, in contrast to the hydrogenation pathways to the carbon atoms C-5 and C-6 of the studied molecules, characterized by the transition states, the hydrogenations at the sulphur atoms for thiouracils are barrierless and thus the latter are the kinetically most favorable pathways for hydrogenation. As also shown, these features are retained under the hydrogenation of the adenine-thiouracil base pairs. The present study clearly demonstrates that the base pairing destruction is the key radiation-induced lesion in the adenine-4-thiouracil and adenine-2,4-dithiouracil base pairs when the hydrogen radical H-., as one of the primary radiation products, resides at the sulphur atom S-4. (C) 2004 Wiley Periodicals, Inc.