This work deals with high-pressure phase behavior of the industrially important polymeric systems polyethylene/ethylene and polybutene/ 1-butene. New experimental data in the pressure range 9 to 17 MPa and the temperature range 405 to 447 K are presented for the system polybutene/1-butene. The polybutene sample has mass-average and number-average molar masses of 1.28 x 10(6) and 0.27 x 10(6), respectively. The polymer concentration ranges from 0.31 to 16.65 mass percent polymer. The system is found to show LCST behavior. The data on the polybutene/1-butene system along with data on mixtures of polyethylene and ethylene reported by de Loos et al. [Th.W. de Loos, W. Poot, G.A.M. Diepen, Macromolecules 16 (1983) 111-117] are modeled using the SAFT and Sanchez-Lacombe (SL) equations of state. Two samples of polyethylene are investigated. One has a mass-average molar mass of 99 000 and number-average molar mass of 56 000 and the other sample has mass-average and number-average molar masses of 8800 and 7600, respectively. Phase equilibrium calculations are performed for two cases: (i) the polymer is treated as monodisperse, and, (ii) the polydispersity of the polymer is accounted for by characterizing the polymer by pseudocomponents. The polydispersity of the polymer has a significant effect on the phase behavior of the systems investigated in this work. Also, sometimes the polymer parameters have to be adjusted to the mixture data to get a reasonable correlation