Segmented polyurethanes (SPU), with oligomeric propylene glycole as a soft-chain component, and 4,4’-diphenylmethane diisocyanate extended with ethylene glycole as a stiff-chain component, were characterized by precise heat capacity measurements in the temperature interval 4.2-420 K. Based on the values of the basic thermodynamic quantities derived from the experimental data, it was concluded that incomplete phase separation resulting in formation of a boundary interphase should be regarded as the main source of intrinsic instability of the three-microphase-type morphology of SPU.