The study of thermodynamic parameters of ice-water phase transition in aqueous solutions of ss-DNA, at different concentration of polynucleotide single chains is reported and the critical concentration for which the enthalpy of transition is zero has been determined. The thermodynamic parameters of ice-water phase transitions, obtained for native ds-DNA and for ss-DNA in the whole concentration region (0.2-2.0 g H2O/g DNA) and the hydration values (bound water quantity) for native (helix), denatured (single strand coils) and unfolded ss-DNA are compared. The critical values of the hydration at which all the water in the DNA-H2O system exists in the bound (unfrozen) state are established, for these conformations, with great precision: N-ds-DNA(Sigma) = (0.55 +/- 0.01) g H2O/g DNA, N-ss-DNA(Sigma) = (0.40 +/- 0.01) g H2O/g DNA. According to this calorimetric data the transformation "double stranded helix" --> "single stranded chains" is accompanied by dehydration of ds-DNA: DeltaN = N-ds(Sigma) - N-ss(Sigma) = (0.15 +/- 0.01) g H2O/g DNA. We suggest that the formation of B-DNA duplex from mixing of their complementary single strands is mostly accompanied by the uptake of structural water molecules and so, the overall hydration of a duplex is mainly determined by its conformation and stability.