A turbidimetric analysis of particle interaction in the presence of dissolved polymer is presented. The system consists of polystyrene latex particles of 75.5 nm diameter and hydroxyethyl cellulose (HEC; M(w) = 172.000 g/mol; radius of gyration = 41.5 nm). To avoid the adsorption of HEC on the surface of the particles, Triton X-405 has been added. The maximum latex concentration investigated was 100 g/L whereas the HEC concentration has been varied between 0 and 0.7 g/L. It can be shown that the scattering in this system is fully determined by the scattering of the latex particles. Turbidimetry, which is insensitive to multiple scattering, allows therefore the determination of the partial structure factor of the latex spheres in the region of long wavelengths. This analysis can be extended to high latex concentrations and yields the osmotic compressibility of the latex spheres. The depletion potential based on a volume-exclusion mechanism is found to provide a good description of the osmotic compressibility.