In recent years, the design of non-viral artificial gene delivery systems has been an important trend in the field of gene therapy. Such systems include the use of copolymer-DNA complexes due to the ionic interactions among the participating species. The resulting complexes are stable in aqueous dispersion, despite complete charge neutralization. To optimize the biological activity of these complexes, it is important to have a complete knowledge of their physicochemical properties. In this work, we report on the interaction of a cationic graft copolymer, poly(ethylene oxide) -g-polyethylenimine (PEO-g-PEI) with poly[d(AT)]. poly[d(AT)] (DNA). A combination of gel electrophoresis, optical, and calorimetric techniques is used to obtain a complete thermodynamic description for both the unfolding of the free and polycation bound DNA, and the interaction of the polycation with DNA. The copolymer-DNA complexes are produced spontaneously resulting from the formation of ion pairs between ionized amino groups of PEI segments of the copolymer and the phosphate groups of DNA. Polycation binding reduces the cooperative unfolding of the DNA without changing the overall conformation of the polynucleotide. The complete thermodynamic profiles show that the interaction of this particular polycation with DNA is generally electrostatic in nature because it exhibits the typical effects induced by increasing the salt concentration. The favorable formation of the polycation-DNA complex is entropy driven and consistent with the observed removal of counterions. The thermodynamic approach taken for this investigation is appropriate, but in order to improve conditions for better DNA delivery systems further investigations of other systems are needed. These systems will have to include variation of the copolymer length, changes in the hydrophilic-hydrophobic balance of the copolymer, as well as the sequence, length, and conformation of DNA.