The DNA thermal denaturation in acidic medium has been investigated experimentally and theoretically. The formulae describing a dependence of the helix--coil transition parameters (melting temperature (Tm) and melting range width (delta T) on ionisation constants values of all kinds of DNA bases in the helix and coil regions and medium acidity have been obtained. Dependences Tm (pH) and delta T (pH) have been determined experimentally and calculated for different models of protonation. Based on the comparison of theoretical and experimental dependences Tm (pH) and delta T (pH) a strict examination of the theory is conducted. The mechanism of DNA protonation is discussed.