The production of cyclic styrene carbonate from CO2 cycloaddition to styrene oxide, using tetrabutylammonium bromide (TBABr) as catalyst, was investigated. Two different reaction systems, a CO2-rich and an epoxide-rich were studied. Reactions were performed in a visual high pressure cell, at different temperatures (313.2, 333.2, and 353.2 K) and pressures (0.5–18 MPa). High pressure phase behaviour manipulation of the reaction system was explored to shift catalysis from homogeneous to heterogeneous, by changing operation conditions. Styrene carbonate formation was monitored quantitatively by using 1H NMR spectroscopy.