Experimental data are presented on the high-pressure phase behavior of 4-methoxyphenylacetic acid (4MPA) with carbon dioxide. High-pressure portions of vapor-liquid, pressure-composition (P-x) isotherms were measured at 68.2, 80.2, and 94.8 degreesC and portions of two solid-fluid isotherms were measured at 35.3 and 52.0 degreesC. The 4MPA + CO2 phase behavior is modeled with the Peng-Robinson equation of state with two mixture parameters fitted to the 80.2 degreesC P-x isotherm. A reasonable fit of the data is obtained if k(ij), the parameter that accounts for specific interactions, is permitted to decrease with decreasing temperature.