Structured micrometric polystyrene/poly(methyl methacrylate) particles were obtained by suspension polymerization and their expansion behavior was investigated using n-pentane as blowing agent. The expanded particles presented two distinct microstructures with an outer region (PMMA-rich shell) composed by cells of about 10 mu m while the center of the particle (PS-rich core) had much larger cells (50-100 m). The core-shell particles did not expand at 100 degrees C meaning that the PMMA shell hindered the expansion of the particles. Maximum expansion was dependent on the PMMA concentration and also on the heating temperature and the increase in the PMMA molar mass led to a delay in the onset of the process. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4521-4527, 2013