When exposed to a dilute solution of free species, the polymer brush functions as a selective barrier to diffusion. Experiments with linear polymer chains and dendrimers of various sizes demonstrated that the selection criterion is relative size, i.e., radius of the free species (radius of gyration for linear chains and simple radius for dendrimers) relative to the radius of gyration of the chains composing the brush. This suggests that linear chains do not necessarily assume extended conformations as they diffuse into a brush but have conformations similar to those of nanoscale spherical inclusions.