3. The analysis of the chemical DNA denaturation process using three-component fractional Hansen parameters shows that hydrogen bonding is the most significant part of the enthalpy of chemical denaturation for T4 bacteriophage DNA. The experimental data for the five fractional cohesion parameters show that the proton-donor effect is the dominant mechanism in hydrogen bonding changes during DNA denaturation. This effect's influence is two times larger than the proton-acceptor effect. Another essential factor for DNA denaturation is the orientational component, part of the polar cohesion parameter. We also suggested that the total cohesion parameter measured at 50% DNA chemical denaturation represents the electrostatic (repulsion) forces maintaining the DNA helix.