The knowledge and the prediction of polymer solutions phase behaviour is of great importance in the designing of polymerization as well as separation processes where liquid- liquid equilibria markedly influence the characteristic of the desired final product. Polymer solutions are complex multi-component mixtures or polydisperse mixtures composed of polymers with different chain lengths and a distributed molecular weight. Continuous thermodynamics method is used to study polydisperse systems due to the large number of components. In this research, stability and phase equilibria of different polymer systems are studied using continuous thermodynamics method. Schulz- Flory distribution function employed to describe different polymers. To study stability and phase equilibria, SWP and Sanchez- Lacombe equations of state are considered. Five different polystyrene-methylcyclohexane solutions, two different polyethylene-ethylene solutions and a polyethylene-n-hexane solution are studied. The monodisperse system of polystyrene in methylcyclohexane shows UCP/LCP behaviour at a certain temperature region. By lowering the temperature, phase diagrams turn into hour glass shaped ones. This behaviour and the effect of polydispersity on stability and phase equilibria of polystyrene-methylcyclohexane solution are modeled using the mentioned equations of state. Both equations of state are able to predict the phase behaviour of the system. SWP equation of state is better in predicting cloud point curves while Sanchez-Lacombe equation of state is preferable in predicting spinodal curves. Polyethylene systems were modeled by Sanchez-Lacombe equation of state. Equality of the chemical potentials and equality of the fugacities method are employed for calculations. Equality of the chemical potentials method is able to predict cloud point curves more accurately in compare with equality of the fugacities method.