The behavior of fluorescent latex probes (radii 0.05, 0.1, and 0.5 mum) in latex host particle suspensions was investigated by fluorescence microscopy with image analysis. The volume fraction of the host latex was varied between 0 and 0.50. A careful statistical analysis was performed to examine the accuracy of the fluorescence microscopy method, from which the direct observation of the Brownian motion gives the diffusion coefficient. The method was found to meet all statistical requirements. From rheological measurements, the maximum volume fraction and the intrinsic viscosity can be obtained. The Krieger-Dougherty equation can be used for the prediction of sample viscosities. The predicted viscosities were used to obtain the theoretical diffusion coefficients with the Stoke-Einstein equation. When comparing the theoretical diffusion coefficients with the experimental ones, it turned out that all models tested yielded acceptable predictions of the diffusion coefficients.