Oligopurine oligopyrimidine sequences of DNA can be recognized by oligonucleotides forming intermolecular triple helices. These triple-helical complexes are strongly destabilized by base pair inversions in the target sequence, i.e., interruption of the oligopurine tract by one or more pyrimidines. Here we show that an acridine derivative incorporated within the sequence of an oligopyrimidine third strand strongly stabilizes the triple-helical complexes. The stability depends on the nature of the base facing the base pair inversion site and the position of the acridine with respect to this base. Fluorescence studies provide evidence that the acridine is intercalated into the tripler. These results show that an extension of DNA target sequences for stable triple helix formation can be obtained with oligonucleotides containing internally-incorporated intercalator(s).