Microcellular foaming of reinforced core/shell Polymethylmethacrylate (PMMA) was carried out by means of supercritical CO2 in a single-step process. Samples were produced using a technique based on the saturation of the polymer under high pressure of CO2 (300 bars, 40 degrees C), and cellular structure was controlled by varying the depressurization rate from 0.5 bar/s to 1.6 x 10(-2) bar/s leading to cell sizes from 1 vim to 200 mu m, and densities from 0.8 to 1.0 g/cm(3). It was found that the key parameter to control cell size was depressurization rate, and larger depressurization rates generated bigger cell sizes. On the other hand, variation of the density of the samples was not so considerable. Low rate compression tests were carried out, analyzing the dependence of mechanical parameters such as elastic modulus, yield stress and densification strain with cell size. Moreover, the calculation of the energy absorbed for each sample is presented, showing an optimum of energy absorption up to 50% of deformation in the micrometer cellular range (here at a cell size of about 5 pm). To conclude, a brief comparison between neat PMMA and the core/shell reinforced PMMA has been carried out, analyzing the effect of the core/shell particles in the foaming behavior and mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 320-331, 2010