Selectivity studies of RNA: disulphide bond containing oligodeoxynucleotide duplexes using melting temperature (Tm) have been reported. The Tm data of the RNA/ DNA duplexes showed that duplex formed with disulphide bond containing oligodeoxynucleotide melted at higher temperature than that of RNA/ unmodified DNA duplex by 9.5℃. Even RNA /oligodeoxynucleotide with adjacent thiol groups formed more stable duplex compared to RNA/unmodified DNA duplex. Studies also showed that the selectivity of duplexes with disulphide bond was out performed duplexes with unmodified oligonucleotide.