The synthesis of thymidine dimers in which the natural phosphodiester linkage has been replaced by piperazine (3 -(N(CH 2 CH 2 ) 2 N)-5 , 9 and 3 -(N(CH 2 CH 2 ) 2 N)-CO-4 ,10 ) are described. These new dimers were incorporated into oligodeoxynucleotides on an automated DNA-synthesizer using the phosphoramidite approach. The thermal stability of DNA/DNA duplexes and the enzymatic stability was studied by UV experiments. 17-Mers with 9 incorporated once or twice in the middle exhibited a pronounced decrease in thermal stability (ΔT m ∼ -11 °C per modification) while 17-mers with 10 incorporated once or twice in the middle exhibited only a slight decrease in thermal stability (ΔT m ∼ -2 °C per modification) when compared to unmodified 17-mers. Furthermore, end-modified oligodeoxynucleotides containing either 9 or 10 displayed five to six fold increased stability towards snake venom phosphodiesterase.