A precipitation process employing 1,1,1,2-tetrafluoroethane (HFC-134a) as antisolvent was studied to see its feasibility for the separation of cycloolefin copolymer (COC) from toluene solution. The precipitation was carried out by spraying toluene solution containing 3 to 12 wt % of COC through a 0.4-mm nozzle into a compressed HFC-134a environment. More than 95% of COC could be precipitated under the conditions that both gas and liquid HFC-134a phases were present in the precipitator. The effects of temperature, pressure, COC concentration, HFC-134a flow rate, toluene solution flow rate, and liquid level in the precipitator on yield and morphology of the precipitated COC were systematically studied. Microspheres of COC with a narrow size distribution were exhibited for the COC concentration equal to or less than 5 wt %. The precipitation at these COC 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 of 8, 10, and 12 wt %, fibers and films instead of microspheres were exhibited. The porosity of the precipitated COC was found to decrease with increasing the COC concentration, indicating that the precipitation was 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, indicating that HFC-134a is an appropriate antisolvent to achieve separation of polymer from solution.