Measurements of the mechanical work (A), the heat of deformation (Q) and differences between these quantities, i.e. the internal energy (ΔU) stored in samples were performed under the unidirectional compression loading conditions by using constant temperature deformation calorimetry. It is shown for several glassy (PS, PC, PI-BD, PET, epoxy-amine network, ABS) semi crystalline (PBT, PET) polymers and blends (PC: ABS, PC: PBT), that 45–85% of the mechanical work of deformation is converted to internal energy stored in deformed samples ΔU is quite high as compared with metals.