Sterically stabilized silica nanoparticles were synthesized, and turbidity measurements confirmed that they behaved as hard spheres in cyclohexane. Poly(isoprene) was added to give mixtures in the protein limit with a polymer coil/colloid radius ratio of 4.8. Their phase behavior under good solvent conditions was studied experimentally. The critical colloid volume fraction was phi = 0.13, whereas recent simulations (Bolhuis, P. G.; Meijer, E. J.; Louis, A. A. Phys. Rev. Lett. 2003, 90, 068304) predicted phi = 0.24. This difference is ascribed to the fact that many systems showing good solvent scaling behavior of the polymer still have a Flory-Huggins parameter close to 0.5, for instance, chi = 0.45 in this work.