A new thymidine analogue, bearing a ferrocenemethyl residue at the N-3 position of the base, was synthesized in high yields via Mitsunobu reaction of ferrocenemethanol with sugar protected thymidine, converted into the corresponding 3'-phosphoramidite and incorporated into oligonucleotides. Duplex and triplex formation experiments, evaluated by UV and CD spectroscopy, showed a dramatic decrease of the affinity towards complementary single strands, while for triplexes, the introduction of a ferrocene residue in the third strand resulted in higher melting temperatures, associated with a reduced content of triplex structure.