The development of predictive equations of state for nanoparticle-surfactant or -polymer mixtures is of extreme importance in nanotechnology and fabrication of advanced materials. Of particular interest is modeling the transitional regime between entropy-con trolled (depletion. repulsive interactions) vs enthalpy-controlled physics (adsorption, attractive interactions). In this paper.. the perturbed Leonard-Jones chain (PLJC) equation of state (EOS)(1) for poly-mer-solvent mixtures is modified and extended to calculate the chemical potentials in nanoparticle-polymer mixtures. The EOS predictions are compared to Monte Carlo simulations that use the same LJ molecular model over a wide range of polymer concentrations approaching the semidilute regime, 0.15 < c(p)/c(p)(*) < 0.8. The original PWG equation. with one adjustable parameter, predicts the nanoparticle chemical potential very well for the enthalpy-dominated strong adsorption. regime, e.g., LJ energy parameters is an element of(cp) > is an element of(pp), where is an element of(cp) = colloid-polymer and is an element of(pp) = polymer-polymer. However, for LJ parameters leading to weak polymer adsorption or depletion, is an element of(cp) < is an element of(pp), the PLJC could not predict simulation results without Further modification. We introduced a semiempirical term that corrects for the polymer-colloid excluded volume. The correction introduces one additional adjustable parameter, but this parameter remained essentially unchanged for all particle compositions, sizes, and is an element of(cp) values studied. These results illustrate that a polymer equation of state. when corrected for the polymer-particle excluded volume, holds promise in modeling the effects of attractive polymeric modifiers on nanoparticle dispersions.