Experimental cloud-point data are reported for polystyrene + methylcyclohexane systems in the vicinity of the upper critical solution temperature (UCST). Two samples of polystyrene were used. One is nearly monodisperse with M-W = 31,600 and M-N = 29,100. The other has M-W = 250,000 and M-N = 64,000. Experimental pressures range from 1 MPa to 14 MPa. Experimental temperatures range from 297 K to 306 K for the lower molar mass polymer in methylcyclohexane and 323 K to 331 K for the higher molar mass polymer in methylcyclohexane. The mass fractions of the polymer range between 0.03 and 0.27 for the monodisperse sample and between 0.02 and 0.14 for the other. Data are also reported on the influence of CO, on the phase behavior of the monodisperse sample in methylcyclohexane for two polymer mass fractions.