The phase behaviors of carbon dioxide (CO2)/tetramethyl orthosilicate (TMOS)/poly(methyl methacrylate) (PMMA) and CO2/TMOS/poly(styrene) (PS) ternary systems were measured. The measurements were performed using a synthetic method involving observations of the bubble point and cloud point. The phase boundaries were measured at temperatures ranging from 313.2 K to 393.2 K and CO2 weight fractions ranging from 0.01 to 0.40. At a constant TMOS:polymer weight ratio of 9:1, both systems showed similar CO2 mass fraction dependence of their phase behaviors. When the vapor-liquid (VL) and vapor-liquid-liquid (VLL) phase boundaries of the ternary systems were compared with those of the binary CO2/TMOS system using polymer-free bases, they were found to be nearly identical. On the other hand, the two systems showed completely opposite polymer mass fraction dependence of their phase behaviors at a constant CO2 ratio.