Cloud-point phase behavior curves to 190 degrees C and 2570 bar are measured for binary and ternary mixtures of poly( isobutyl acrylate)-CO2- isobutyl acrylate, poly(tert-butyl acrylate)- CO2- tert- butyl acrylate, poly( isobutyl methacrylate)- CO2- isobutyl methacrylate, and poly( tert- butyl methacrylate)- CO2- tert- butyl methacrylate systems. The phase behavior for the poly( isobutyl acrylate)- CO2- isobutyl acrylate system with cosolvent concentrations of 0, 3.2, 8.8, 18.1, 31.9, and 40.7 wt % are measured in the changes of the pressure-temperature slope. With 58.7 wt % isobutyl acrylate to the poly( isobutyl acrylate)- CO2 solution significantly changing, the phase behavior curves takes on the appearance of a typical LCST boundary. The cloud-point curves for the poly( tert- butyl acrylate)-CO2- 0, 8.5, 14.9, 34.7, 56.0, and 57.7 wt % tert- butyl acrylate system changes the P-T curve from the UCST region to the LCST region as the tert- butyl acrylate concentration increases. The cloud-point curves for the poly( isobutyl methacrylate)- CO2- isobutyl methacrylate system show the slope changes of the pressure-temperature diagram and cosolvent concentrations of 0, 6.5, 12.6, 21.7, and 32.3 wt %. With 49.8 wt % isobutyl methacrylate added to the poly( isobutyl methacrylate)- CO2 solution, the phase behavior curves show a typical LCST boundary. And also, the cloud-point curves for the poly( tert-butyl methacrylate)- CO2- 0, 10.2, 20.5, 31.3, and 46.4 wt % tert-butyl methacrylate system change the P- T curve from the UCST region to the LCST region as the tert- butyl acrylate concentration increases. High-pressure vapor-liquid equilibria are obtained for the CO2-isobutyl acrylate, CO2- tert- butyl acrylate, CO2- isobutyl methacrylate, and CO2- tert- butyl methacrylate systems at the range of temperature of 40120 C and pressure up to 160 bar. The CO2- isobutyl acrylate, CO2- tert- butyl acrylate, CO2- isobutyl methacrylate, and CO2- tert- butyl methacrylate systems exhibit type-I phase behavior with a continuous mixture-critical curve. Three-phase, liquid-liquid-vapor equilibrium was not observed at these conditions. The experimental results for CO2- isobutyl acrylate, CO2- tert- butyl acrylate, CO2- isobutyl methacrylate, and CO2- tert- butyl methacrylate mixtures are modeled using the Peng-Robinson equation of state.