In this work, a novel method to simultaneously measure in situ swelling and sorption of gases and supercritical fluids in polymers in a commercially available apparatus is proposed. A gravimetric approach reported recently is applied to measure sorption whereas swelling is determined in situ by direct visualization. The proposed method has been used to study the sorption and swelling of carbon dioxide in poly(methyl methacrylate) and the obtained results are compared to previously published literature data. (c) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 574-580, 2011