The interaction of DNA with a synthetic biocompatible, and biodegradable cationic polymer, poly(methylaminophosphazene) hydrochloride (PMAP HCl); was investigated by high-sensitivity differential scanning calorimetry under conditions of strong and weak electrostatic interactions of the macroions. Thermodynamic parameters of the DNA double-helix melting were determined as a function of pH and the PMAP center dot HCl/DNA weight ratio., PMAP center dot HCL was shown reveal, two functions, with respect to DNA the polyelectrolyte function and the donor-acceptor one The first function stabilizes the helicgconformation of T, C DNA, and the second one destabilizes it The stabilizing effect of PMAP center dot HCl is of entropic origin, related to a displacement of mobile counterions from the DNA's nearest surroundings by the poly(methylaminophosphazene) charged groups. The donor-acceptor function of poly(methylaminophosphazene) dominates when its electrostatic interaction with DNA is either saturated (in the complex coacervate phase at high poly(methylaminophophazene) concentrations) or completely suppressed (in a salt medium when the polycation carries a small charge). Under these conditions,, poly(methylaminophosphazene) destabilizes DNA. It preferentially binds to the DNA coil form likely via the formation of multiple labile hydrogen bonds with the donor acceptor groups of DNA.