In this paper a novel method to simultaneously measure swelling and sorption of supercritical fluids in polymers in a commercially available setup is proposed. A purely gravimetric approach based on the use of a magnetic suspension balance is adopted to study the swelling/sorption behavior of poly(methyl methacrylate) in an atmosphere of CO2. The procedure to determine swelling and sorption involves two steps, one using pure CO2 and the other adding an inert to the system. From these two measurements, the swollen volume and the amount Of CO2 sorbed in the polymer can be calculated on the basis of a number of assumptions whose validity is critically discussed. To test the reliability of the new protocol and to determine the recommended operating window, the obtained results are compared with measurements taken with a standard technique as well as literature data. It is concluded that the proposed technique works in a window of operating conditions that is system specific. This is due to a strong sensitivity of the adopted equations to the small effect of the inert on the sorption/swelling behavior of the system.