We have used neutron reflectometry to characterize the swelling behaviour of brushes of poly[2-(diethyl amino)ethyl methacrylate], a polybase, as a function of pH. The brushes, synthesized by the "" method of atom transfer radical polymerization, were observed to approximately double their thickness in low pH solutions, although the p is shifted to a lower pH than in dilute solution. The composition-depth profile obtained from the reflectometry experiments for the swollen brushes reveals a region depleted in polymer between the substrate and the extended part of the brush.