Equilibrium melting curves were obtained for triplexes, formed by single stranded DNA containing an A₁₀ target with bis-PNA consisting of two T₁₀ decamers. Thermodynamic parameters of melting were determined for Na⁺ concentrations 50, 200 and 600mM by two methods. The melting enthalpy ΔH° was evaluated from the width of the differential melting curves and from the concentration dependence of the melting temperature. The latter method allowed also evaluating the melting entropy ΔS&deg. The following results were obtained: ΔH&deg = -137 kcal/M, ΔS&deg = -368 cal/M·K, ΔG&deg₂₉₈= -27 kcal/M. No dependence of ΔH&deg, ΔS&deg and ΔG&deg₂₉₈ was observed upon ionic strength within the accuracy of the experiment (±10%). The absolute values of ΔH&deg, ΔS&deg and ΔG&deg₂₉₈ are 2 to 3 times higher than the published values of corresponding melting parameters for decameric PNA/DNA duplexes of various nucleic base sequences. The origin of the extremely high stability of the triplexes is discussed.