Using the free volume theory, general relations between viscometric and volumetric properties of polymers as functions of temperature and deformation conditions (shear rate or shear stress) have been derived. In particular, it has been shown that thermal expansion of polymers can be calculated from viscometric data. The obtained relations have been applied for description of the rheological behavior of polymer blends. Theoretical predictions have been compared with experimental data. Good agreement between theory and experiment has been stated if rheological properties of blends were compared at a constant shear rate.