The present work investigates the structure of large polysstyrene (PS) chains in two different solvents in the presence of octa-n-propylsilsesquioxane (SILS) molecules. Both sohent, are good Solvents for the PS chains. Viscosity measurements combined with static and dynamic light scattering revealed a significant shrinking of the PS chain dimensions, with increasing the SILS concentration. At the same time, chain-chain interactions decreased. Highly significant light scattering experiments became possible due to the fact that one of the solvents provided a refractivity which matched the scattering contrast of SILS, thus giving way to an unperturbed picture of the polymer chains. Together with the evaluation of the phase behavior of the PS-SILS mixture in toluene and in a theta-solvent. these results suggest that the PS-SILS mixture represents an interesting model system for colloid-polymer mixtures in the prottein limit. Supplementing SANS experiments on SILS in solutions of perdeuterated toluene with perdeuterated PS chains clearly indicated a zone depleted of SILS. In line with this, the crystallization tendency of SILS was increased with increasing PS concentration due to a condensation of SILS in polymer-free domains.