AbstractCationic model compounds of polynucleotide,i.e., poly{1‐[2‐(adenin‐9‐yl)ethyl]‐4‐pyridinioethylene chloride} (1), poly{1‐[2‐(thymin‐1‐yl)ethyl]‐4‐pyridinioethylene chloride} (2), and poly{1‐[2‐(theophillin‐7‐yl)ethyl]‐4‐pyridinioethylene chloride} (3), were synthesized by the MENSCHUTKIN reaction of poly(4‐pyridylethylene) with 9‐(2‐chloroethyl)adenine, 1‐(2‐chloroethyl)thymine and 7‐(2‐chloroethyl)theophylline.The interactions between the model compounds and polynucleotides and between the model compounds themselves were studied by spectroscopy. The apparent hypochromicity was about 20% for the mixture of1‐Poly A, 15% for that of1‐DNA and 11% for the1–3mixture. The systems1–2and2–3showed rather small hypochromicities, being 8 and 6%, respectively. The hydrogen bond formation between complimentary bases is not a main factor in the intermolecular interactions in the present cases, but the hydrophobic interactions are suggested to be overwhelming. The UV absorbance of1–2or2–3mixtures decreased with increasing temperature