We have continued our theoretical study of 5-substituted-2'-deoxyuracils, e.g I (R=Et, CHME2, CH:CH2, C.tplbond.CH, CH2OH, CH2SMe), with known antiviral activity, and have developed a comparative structural study of 21 structures whose geometries were calculated by molecular mechanics (MM) and molecular orbital (MO) (using the AM1 and the PM3 hamiltonians) techniques. The results have been compared with several 5-substituted-2'-deoxyuridine crystal structures extracted from the Cambridge Structural Database (CSD) and with the ab initio results for uracil itself. The conformation of the furanose ring and the relative disposition of the C4'-C5' side-chain are discussed. The electrostatic potentials in the surroundings of certain atoms have also been investigated.