The intramolecular hydrogen bond between the adenine and thymine rings of 9-[omega-(thymin-1-yl)alkyl]adenine was investigated in chloroform by means of NMR spectroscopy. The formation of the intramolecular hydrogen bond was dependent on the length of the polymethylene chains between adenine and thymine, and the thermodynamic parameters were estimated. The length of the polymethylene chains caused a marked difference in the physical properties of the compounds.