For a number of organic glassy polymers with different chemical structures and immiscible blends, the mechanical work expended for deformation, the plastic deformation heat and the internal energy stored in deformed samples (stored energy of cold work) have been measured using different experimental techniques. A comparison of the measured quantities is made and suggestions concerning their nature are discussed. it is found that the fraction of expended mechanical work transformed into stored internal energy upon deformation is very high for all studied systems. This reflects the non-isostructural character of inelastic deformation in glassy polymers. A two-stage deformation mechanism is introduced and the experimental results are analyzed in a framework of the suggested mechanism.