Using the quartz crystal microbalance, we studied the formation of polymer brushes and the penetration of these brushes by chemically identical free chains. The systems studied were composed of thiolend-functionalized polystyrene chains, toluene, and gold. Brushes were exposed to dilute solutions of free chains of molecular weight both greater than and less than that of the chains of the brush. Only the free chains of lower molecular weight penetrated the brush. These chains diffused all the way to the underlying gold surface and became grafted, while chains of larger molecular weight did not penetrate the brush to any extent whatsoever.