The liquid-liquid equilibrium (LLE) and interfacial tensions of binary and ternary polymer solutions of polystyrene in cyclohexane and methylcyclohexane are investigated. The chain length dependent modified double lattice model (MDL-CL) is applied to describe experimental phase diagrams measured by thermo-optical analysis. The proposed model is also combined with the modified density gradient theory to calculate interfacial tension between two coexisting liquid phases of the investigated systems. The calculated results of LLE and interfacial tensions using the MDL-CL model combined with the modified density gradient theory show good agreement with experimental data. The interfacial tension of the ternary system (polystyrene + cyclohexane + methylcyclohexane) is described with the parameters obtained from the binary subsystems (polystyrene + cyclohexane and polystyrene + methylcyclohexane). A comparison of experimental and theoretical phase behaviors and interfacial tensions demonstrates that the theoretical approach is able to describe the experimental result with a few adjustable parameters.