Double-stranded oligonucleotides of different lengths and chemical modification have been analyzed by ion spray mass spectrometry. The non-covalent-bonded duplexes can be detected. Therefore, ion spray mass spectrometry is a useful method for investigation of hybridizations of natural and chemically modified oligonucleotides. Since the exact mass of the double strand can be detected, this method can distinguish between specific and nonspecific interaction.