In order to analyse the effect of supercritical fluid (SCF) addition on polymer viscosity we have used, for the first time, a high precision vibrating wire instrument, for the simultaneous measurement of viscosities and densities of a SCF-saturated polymer. For the density measurements, this technique makes use of the buoyancy force exerted by a fluid on a solid sinker, detected by means of a vibrating wire sensor placed inside the measuring cell. The viscosity and density of the fluid are obtained by analysis of the resonance curve of the vibrating-wire, using a rigorous model for describing the hydrodynamic effects of the fluid on the wire motion. Results of the preliminary measurements carried out on a commercial linear thermoplastic oligomer [Poly (ethyleneglycol) 400] saturated with CO2 are presented, in a range of temperatures from 313 to 348 K (3 isotherms) and at pressures up to 25 MPa. A sharp decrease of the viscosity was observed for all temperatures. The highest viscosity reduction was reached at the lowest temperature.