Various types of liquid liquid equilibrium of polydisperse polymer solutions, including upper critical solution temperature (UCST), both UCST and lower critical solution temperature (LCST), and closed loop miscibility, are investigated. Determination of the cloud-point curves for polydisperse polystyrene (PS)/cyclohexane and PS/methyl acetate systems is conducted by thermal optical analysis. In addition, the molecular weight distribution of polydisperse PS is determined by a gel permeation chromatography technique. To describe the phase equilibria of polydisperse polymer solutions, a continuous thermodynamic approach is incorporated into the incompressible lattice model. This appropriately describes the characteristic phase behaviors of polydisperse polymer solutions, and the calculated results show good agreement with the experimental data.