This paper deals with experimental and theoretical investigations on the compatibility of binary polymer blends in solution. The experimental phase boundary of a crystalline polymer such as poly(vinylidene fluoride) with polystyrene (an amorphous polymer) in dimethylformamide as solvent has been determined by size-exclusion chromatography at 25-degrees-C. The composition of the coexisting phases has been used to calculate interaction parameters by means of the Flory-Huggins-type function for the free energy of mixing, including composition dependence of the polymer-polymer interaction parameter and a first correction term denoted as ternary interaction parameter. The calculation of these parameters involves the solution of a system of equations that should be equal to the number of unknown parameters. Diverse approximations have been proposed in order to reduce the number of equations to be solved; they are mainly concerned with the composition dependence of both binary polymer-polymer and ternary interaction parameters and of their derivatives.