Anthraquinone and pyrene analogues attached to the 3' and/or 5' termini of triplex-forming oligonucleotides (TFOs) by various linkers increased the stability of parallel triple helices. The modifications are simple to synthesize and can be introduced during standard solid-phase oligonucleotide synthesis. Potent triplex stability was achieved by using doubly modified TFOs, which in the most favourable cases gave an increase in melting temperature of 30 degrees C over the unmodified counterparts and maintained their selectivity for the correct target duplex. Such TFOs can produce triplexes with melting temperatures of 40 degrees C at pH 7 even though they do not contain any triplex-stabilizing base analogues. These studies have implications for the design of triplex-forming oligonucleotides for use in biology and nanotechnology.