The effect of imidazo[4,5-d]phenazine (Pzn) attached to the 5(')-end of (dT)(15) oligonucleotide via a flexible linker on the thermal stability of poly(dA)·(dT)(15) duplex was studied in aqueous buffered solution containing 0.1 М NaCl at the equimolar ratio of adenine and thymine bases (100 μM each) using spectroscopic techniques. Duplex formation was investigated by measuring UV absorption and fluorescence melting curves for the Pzn-modified system. Tethered phenazine derivative enhances the thermostability of poly(dA)·(dT)(15) duplex increasing the helix-to-coil transition temperature by 4.5 °С due to an intercalation of the dye chromophore between AT-base pairs. The thermodynamic parameters of the transition for non-modified and modified systems were estimated using "all-or-none" model. The modification of the (dT)(15) results in a decrease in the transition enthalpy, however, the observed gain in the Gibbs free energy of complex formation, ΔG, is provided with the corresponding decrease in entropy change. The increase of ΔG value at 37 °C in consequence of (dT)(15) modification was found to be equal to 1.3 kcal/mol per oligonucleotide strand.