Extruded microcellular foams have been obtained from mixtures of polycarbonate (PC) and n-pentane. Cell diameters were in the range of 2 to 5 mum and the foam densities varied between 400 and 700 kg/m(3). Although two types of PC have been investigated, one linear and one branched, the presence of side branchings did not modify the extruded foam characteristics. Use of carbon dioxide as the blowing agent was also attempted, and cell sizes below 10 mum have been successfully obtained. One prerequisite for microcellular foaming was believed to consist in a concentration of the blowing agent close to its limit of solubility as that defined under the actual processing conditions of pressure and temperature. This hypothesis was validated from the observation of extrusion of regular PC foams (intermediate to low densities and cell sizes ranging between 100 mum and 1 mm) using moderate concentrations of blowing agents, and from solubility and viscosity measurements on similar polymer/blowing agent systems.