3. Analysis of the chemical DNA denaturation process using Hansen-fractional threecomponent parameters shows that hydrogen bonding is the most significant part of the enthalpy of chemical denaturation for the DNA of the T4 bacteriophage. 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 denatura-tion. The influence of this effect is two times greater than that of the proton-acceptor effect. Another essential factor for DNA denaturation is the orientation component, which is 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 that maintain the DNA helix.