Dynamic light-scattering data for polystyrene fractions in a good solvent (benzene) and a theta solvent (cyclopentane, 20.4°C) are presented. They encompass a broad concentration interval in the semi-dilute range. These data are compared with those for the systems poly(ethylene oxide)/water and hydroxyethylcellulose/water. The entanglement network in these systems is reflected in part by the nature of the concentration dependence of the mutual diffusion coefficient and in part by the presence of a slow relaxation, information on which may be derived from the non-exponential autocorrelation function.