Microcellular plastics are foamed polymers of cell size smaller than 10 μm. The development of manufacturing processes fcr microcellular plastics strongly depends on the sorption parameters of the blowing agent in the polymer. The goal of this research is to design a sorption apparatus for measuring the solubility and diffusivity of liquid blowing agents in polymers at an elevated pressure. A sorption experiment measures the mass gain of the polymer due to the permeation of the blowing agent in the polymer. In this thesis, a constant pressure sorption apparatus was developed to correlate the volume change of the blowing agent to the mass gain of the polymer. As a case study, an investigation was performed using isopentane and high impact polystyrene as the blowing agent and the polymer, respectively. Although the sorption apparatus successfully measured a volume change due to polymer sorption, the entire polymer mass gain could not be determined due to polymer swelling. It is concluded that the in-situ measurement of the amount of liquid permeated into the polymer can be achieved by monitoring the degree of swelling in a modified sorption device.