The theoretical approach to the calculation of the influence of selective binding of small ligands on DNA helix-coil transition has been described in the previous paper (Lando D. Yu., J. Biomol. Struct. Dyn., (1994)). In the present paper that method is used for the study of DNA protonation and deprotonation in acidic and alkaline medium by theoretical analysis of pH effect on DNA heat denaturation. The mechanism of DNA protonation in acidic medium and pK values of nucleotides are well known. It gave us an opportunity to check the theory without any fitting of pK values. A good agreement between experimental and calculated functions Tm(pH) and delta T(pH) (melting temperature and melting range width) obtained for acidic medium proved the validity of the theory. However, for alkaline medium there was not even qualitative agreement when the agreed-upon mechanism of deprotonation was considered. Looking into the cause of the discrepancy, we have studied the DNA melting for different mechanisms of deprotonation by calculation of Tm(pH) and delta T(pH). As a result, it has been established that the discrepancy is due to deprotonation of bonded GC base pairs of helical DNA regions (pK = 11). It was shown that the early known protonation and newly found deprotonation of helical DNA essentially stabilised double helix in alkaline and acidic medium.