The Brownian motion of carboxylated polystyrene latex, labeled with fluorescent probe molecules, at low concentrations in aqueous solutions was investigated by dynamic fluorescence microscopy. For all three latex radii, i.e., 0.5, 0.265, and 0.1 mum, the estimated diffusion coefficients correspond well with the theoretical predictions if thermal and electrostatic contributions are included in the discussions. It was also possible to discriminate latex interactions with an added polymer. Added polyethylene glycol showed no or very weak interaction with the latex until the polymer overlap concentration was reached, at which the formation of a polymer network slowed down the latex diffusion. Polyvinylpyrrolidone, on the other hand, had a more pronounced interaction with the polystyrene latex and slowed down the diffusion even at polymer concentrations in the ppm range. The overall conclusion is that fluorescence microscopy is a feasible method for the study of the dynamic behavior of small particles in solution. (C) 2003 Elsevier Inc. All rights reserved.