This paper deals with the gas absorption behavior of polymer systems. The emphasis is on the difference between filled and unfilled polymers to explain heterogeneous nucleation in filled polymers. A Foaming Process Simulator has been built to study the gas absorption. It consists of a test chamber that holds the polymer samples. The chamber can be pressurized with gas up to 5000 psi and heated up to 450 degreesF. The gas pressure is monitored by a high-accuracy pressure transducer and recorded by a data acquisition system. The amount of gas absorbed by a polymer is determined from the pressure change in the test chamber. A rotor applies shear to the polymer melt to investigate the shear effects. Two polymer systems were tested: HDPE with/without talc, and PVC with/without calcium carbonate. The fillers were well dispersed as verified with scanning electron microscope. The tests were done at the melt temperatures from 120 degreesC to 177 degreesC and a saturation pressure of about 18.9 MPa. It was found that the filled polymers absorb more gas compared to the unfilled ones. It is suggested that there is a certain amount of gas accumulated in the filler-polymer interface. This accumulated gas helps to create cells during the foaming process.