Polystyrene (PS) foams are commonly used as heat insulators. Their heat insulation properties can be greatly improved by decreasing the cell size into the range of micrometres, resulting in the so-called micro-cellular foams. We prepared micro-cellular PS foams and studied the influence of toluene residua on the foam structure. First, PS films were prepared by the dip-coating method using toluene as the solvent. Then, the PS films were foamed by the pressure induced foaming method using high pressure CO2. The foam structures were examined by Scanning Electron Microscopy and X-ray micro-tomography. For the industrial PS sample containing nucleation agents, the influence of temperature and depressurization rate on cell size was observed in a way consistent with the literature. However, the obtained cell sizes were much larger. Therefore, the effect of toluene residua on the foaming process was systematically studied. A PS sample without nucleation agents was used to exclude their effect on the foaming process. Toluene acts as a co-solvent and enhances the CO2 solubility in PS, which should lead to a large amount of small cells. However, at toluene concentrations below 8 %wt, the plasticizing effect of toluene dominates, resulting in enhanced coalescence and cell growth. Foam heat insulation properties improve with increasing porosity. The toluene residua increased the porosity by increasing the cell sizes and lowering the thickness of the compact skin at the film surface. We conclude that toluene residua greatly influence the foam structure and this finding can be potentially used to control the foaming process. [ABSTRACT FROM AUTHOR]