The viscosity of molten polymers depends on the processing conditions such as shear rate, temperature and pressure. The pressure dependency is often ignored, although it can be important in polymer processing operations such as injection moulding and extrusion. The main reason for the limited attention for pressure effects is the fact that viscosity measurements at high pressure are quite intricate. Thus, establishing alternative methods to assess the pressure dependency of the viscosity can facilitate and improve numerical simulations for the design and optimization of polymer processing equipment. It has been demonstrated in literature that variations in viscosity with either pressure or temperature can be related to changes in the polymer free volume. Nevertheless, more systematic investigations are needed to unambiguously correlate free volume on the one hand and pressure and temperature dependent viscosities on the other hand. In the present work, miscible PMMA/PVDF blends of different compositions have been prepared by melt extrusion to allow to systematically vary the free volume and temperature and pressure coefficients of viscosity. The melt viscosity at different temperatures and pressures was measured by means of a capillary rheometer equipped with a specially designed pressure chamber. Based on these data, temperature and pressure coefficients of viscosity at constant shear rate and at constant shear stress were determined. Pressure-volume-temperature data were collected by means of a high pressure mercury dilatometer. From the PVT data in the melt region, the free volume of the materials was extracted by means of fits with the Simha-Somcynski equations of state. Interestingly, unique linear relations were found between the temperature coefficients of viscosity and the pressure coefficients of viscosity as well as between the pressure coefficients obtained from PVT data and the rheologically determined pressure coefficients.