Triple helical structures can be observed between double-stranded nucleic acids and a third strand through the formation of Hoogsteen hydrogen bond. We report here the use of quinazoline-2,4-dione derivatives as substitutes for thymine in TA*T triplets. The synthesis and the characterization of monochloro derivatives of quinazoline-2,4-dione as well as 5-fluoro and 6-nitro substituted quinazoline rings are described. The ability of the various modified bases to promote the formation of triplexes was reached by thermal denaturation studies. (C) 1997 Elsevier Science Ltd.