Using emulsifier-free emulsion polymerization method, monodispersed crosslinked poly(styrene-co-methacrylic acid-co-ethylene glycol dimethacrylate) colloid microspheres were synthesized. The microspheres were treated in supercritical carbon dioxide (SCCO2) after they have self-assembled into ordered three-dimensional (3D) colloid arrays. The CO2 absorbed into the polymer microspheres enhances chain segments mobility and reduces the glass transition temperature (T-g) of polymer, which induce the microspheres coalesce at the relatively low temperature. The coalescence degree of microspheres was studied by varying the experimental temperature, pressure, and exposure time in SCCO2. The results were shown by scanning electron microscopy (SEM). Further, the lattice spacing of the assembled 3D colloid microspheres was calculated from the SEM images. It was illustrated that the coalescence degree enhanced with the increase of CO2 pressure; however, this tendency became weak when CO2 pressure reached a certain value. Extending exposure time in SCCO2 or elevating temperature can also increase coalescence degree, and the effect of temperature is more significant. It is believed that these results will make sense when the polymer microspheres are considered to be used as templates in SCCO2; meanwhile, it raises a new method about tuning the final morphology of the stabilized colloidal crystals and porous materials via controlling the coalescence degree with the assistance of SCCO2. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2393-2399, 2011