We have generated microcellular polymeric foam structures using a pressure induced phase separation in concentrated mixtures of supercritical CO2 and poly(methyl methacrylate). The process typically generates a microcellular core structure encased by a nonporous skin, the thickness of which decreases with increasing saturation pressure. This trend can be described by a model for skin formation that is based on the diffusion rate of gas out of the sample. Significant density reductions on the order of 30 to 70% can be achieved by changing the pressure and temperature conditions in the foaming process. There are several ways in which the saturation pressure affects the average cell size, with the net effect that cell size decreases sharply with increasing pressure above 2000 psi, leveling out at higher pressures. Cell size increases with increasing temperature from 40-degrees to 70-degrees-C. A model for cell growth, based on a cell model of Arefmanesh and Advani, modified to include the effect of CO2 on model parameters. reproduces these trends.