The chain length and compositional dependence of the effective chi parameter, chi(s), in model isotopic polyethylene blends were determined at two different temperatures using our recently developed optimized cluster theory For polymer mixtures. chi(s), was found to have a concave-up compositional dependence that decreased with increasing temperature and molecular weight, in agreement with neutron scattering measurements. A best fit to the chain length dependence of chi(s) at constant temperature and composition had the form, chi(s) = chi(s)(infinity) + D/N, where N is the number of segments comprising the polymers. In addition, the calculated compositional dependence of chi(s) could be accurately described by a simple functional form that was previously proposed to fit experimental data on isotopic blends. However, the theory underestimates the breadth of the compositional dependence reported in the literature.