The two-phase boundaries of mixtures of carbon dioxide and 1,2-epoxycyclohexane (cyclohexene oxide) were determined at 343 K, 363 K, 383 K, and 405 K for cyclohexene oxide mole fractions of 0.02 < x(2) < 0.65. The bubble/dew points were determined visually in a windowed, variable volume, high-pressure cell during isothermal expansions and compressions of mixtures of known overall composition. Pressure-composition diagrams were constructed from the experimental data, and the critical points of the mixtures were determined for each temperature from these phase diagrams. Although the Peng-Robinson equation of state slightly overestimated the critical pressure, an excellent fit of the remainder of the bubble and dew point data was obtained with a binary interaction parameter of 0.05. Anomalous phase behavior, in the form of remarkably high dew point data, was observed at 383 K and 405 K. H-1 NMR results of the cyclohexene oxide from these experiments suggest that this was due to the formation of poly(cyclohexene ether). The formation of this polymer may have been catalyzed by trace amounts of water in the system. This polymer has a low solubility in CO2; therefore, its presence increased the dew point pressure.