Communication: The role of activator and deactivator species in the surface-initiated atom-transfer radical polymerization of styrene using CuBr/CuBr2/pentamethyldiethylenetriamine as a model system is described. The influence of initially added deactivator with respect to the degree of controlling the layer growth and thickness is studied. Variation of the activator concentration results in changes of the kinetics as well as brush thicknesses consistent with the well-known rate laws of ATRP.