We have used neutron reflection to study the effects of pH and added electrolyte on the structure of a weak polyelectrolyte brush anchored by means of a hydrophobic block to a hydrophobic surface. The copolymer was poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) (poly(DMAEMA-b-MMA)) containing 70% DMAEMA and having a molecular weight of 10000. The MMA block and the hydrophobic octadecyltrichlorosilane (OTS) layer were deuterated in order to highlight the DMAEMA fraction of the layer. The MMA block was found to bind strongly and irreversibly to a self-assembled monolayer of octadecyl trichlorosilane (OTS) on silica. The copolymer distribution could be described in terms of two uniform layers, a thin layer next to the surface containing all the MMA block and variable amounts of the DMAEMA residues with a copolymer volume fraction of about 0.8 and a more diffuse block corresponding to the polyelectrolyte brush. The presence of DMAEMA residues in the thin layer and the variation of this amount with pH and added electrolyte show that, near the anchor layer of the MMA block, the DMAEMA block is not ionized, in agreement with the model of a grafted weak polyelectrolyte proposed by Israels et al.