The compressed CO2 was observed to be an effective antisolvent to separate cycloolefin copolymer (COC) from toluene solution. The precipitation was carried out by spraying a toluene solution containing COC into a compressed CO2 environment. It was found that more than 95% of the dissolved COC could be precipitated at the conditions that both gas and liquid CO2 phases were present and the liquid level was located at the middle in the precipitator. Submicron-sized spheres of COC with a narrow size distribution were exhibited for the COC concentrations less than 6 wt%. The precipitation at these concentrations followed a nucleation and growth mechanism. Nucleation and solution breakup that affected morphology and size of the precipitated COC were allowed to occur in the gas space above the liquid. For the concentrations equal to and higher than 7 wt%, continuously interpenetrating structures instead of spheres and a decrease in porosity of the precipitated COC with increasing the COC concentration were exhibited, indicating the precipitation under a spinodal decomposition mechanism. The molecular weight and glass transition temperature of the precipitated COC were found to be sufficiently close to those of the virgin chips.