The diffusion of rhodamine-labeled poly-(ethylene glycol) (r-PEG) within surface-grafted poly-(ethylene glycol) (s-PEG) layers in aqueous solution at 18 degrees C was measured by fluorescence correlation spectroscopy. The diffusion coefficient of r-PEG within s-PEG was controlled by the grafting density, sigma, and scaled as sigma(-1.42 +/- 0.09). It is proposed that a characteristic blob size associated with the grafted (brush) layer defines the region through which the r-PEG diffusion occurs. The diffusion coefficients for r-PEG in semidilute solution were found to be similar to those in the brushes.