A perturbation theory equation of state for mixtures of freely jointed square-well fluids of variable well width that takes into consideration the influence of the attractive part of the intersegment potential on the chain properties is applied to describe thermodynamic properties of pure normal fluids, polymers, and their mixtures. The equation of state is based on second-order Barker and Henderson perturbation theory to calculate the thermodynamic properties of the reference sphere fluid and on first-order Wertheim thermodynamic perturbation theory to account for the connectivity of spheres to form chains. Pure polymer parameters are determined from low molecular weight pure substance parameters, from pure polymer volumetric data, and from mixture data. Good results are obtained when this equation of state is used in the calculation of vapor-liquid equilibrium data of highly asymmetric mixtures, with polymer properties determined from low molecular weight substance properties, molecular weight and temperature-independent parameters, and no binary adjustable parameters.