Tracer diffusion coefficients for star-branched polystyrene diffusing into linear, intramolecularly cross-linked (microgel), and intermolecularly cross-linked (network) polystyrene matrices were measured by forward recoil spectrometry. The diffusion behavior can be modeled by assuming that constraint release and arm retraction mechanisms are both operating. Microgels and networks behave as if they are linear chains of infinite molecular weight, so that only the arm retraction contribution is measured. It is found that, for a given arm molecular weight, four-and eight-arm stars diffuse about as quickly as three-arm stars in microgel matrices. A simple arm retraction mechanism is proposed whereby diffusive steps occur each time a single arm retracts, giving rise to a relatively weak dependence of the diffusion coefficient on the number of arms of the star. © 1988, American Chemical Society. All rights reserved.