Equilibrium and dynamic sorption isotherm measurements, differential scanning calorimetry (DSC) measurements, and, mainly, dielectric relaxation spectroscopy (DRS) measurements by means of the thermally stimulated depolarization currents (TSDC) method were used to investigate the hydration properties of linear segmented polyurethane copolymers. Three types of samples were investigated with various fractions of hard and soft block segments. They were based on polyethylene adipate (PEA), 4,4’-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO). At 20 degrees C the water content h of the samples at various values of relative humidity rh increases in proportion to the weight fraction of soft block segments phase. At saturation (rh = 100%) the ratio of sorbed water molecules to polar carbonyl polyester groups is 0.13. At saturation at 20 degrees C there is no fraction of freezable water. The glass transition temperature, T-g, measured by DSC and by TSDC, shifts to lower temperature with increasing h by about 8-10 K at saturation at 20 degrees C. A dielectric relaxation mechanism related to interfacial polarization in the phase-separated morphology is also plasticized by water in a way similar to that observed for the main (alpha) relaxation.