The phase behavior of a molecular-based model of nonpolar binary mixtures is investigated by the application of thermodynamic perturbation theory. The pair potential interaction model is represented as a reference system of equal size hard-spheres perturbed by the exponential-6 potential. It is found that the qualitative behavior of gas-gas immiscibility of second kind is very much sensitive to the hardness of the potential (ratio of the hard sphere diameter to the size parameter of the perturbation). Calculated theoretical phase diagrams are compared with results from Monte Carlo simulations in the Gibbs ensemble performed for this model mixture. (C) 1997 American Institute of Physics.