The enthalpy for thermal denaturation is positive (endothermic process), and the enthalpy of DNA melting for chemical denaturation is negative (exothermic process). Experimental results show that the absolute enthalpy values for DNA chemical denaturation are significantly lower than in the thermal denaturation process. The melting temperature for DNA thermal denaturation is lower than for chemical denaturation in the same system, indicating that the mechanism of reaching 50 % DNA denaturation thermally and chemically is different.