Decompression experiments from a PMMA compound saturated with CO2 at high pressure were performed to investigate the heterogeneous effect due to the aggregate of the additives on the microcellular foam structure generated. The observed results were compared with calculation results from a previously proposed homogeneous nucleation model. Uniform distribution of foams was obtained at a lower temperature of T = 20 degrees C at various initial pressures tested. However, the heterogeneous effect due to the aggregate of the additives in the compound occurred at the solution temperature of T = 40 degrees C, which is quite different from the results obtained in PMMA/ CO2 solution where homogeneous nucleation occurs in the temperature range between 40 degrees C and 70 degrees C Experimental results obtained in this study may be applied to the injection-molding process of PMMA compounds.