Poly(methacrylic acid) brushes have been prepared utilizing the "grafting from" technique and a living radical synthesis route using a two stage process. Firstly a poly(1-ethoxyethyl methacrylate) brush was synthesized by atom transfer radical polymerization and then thermally decomposed to poly(methacrylic acid). The swelling behaviour of the weak polyacid brush was investigated as a function of pH and salt concentration in aqueous solutions using atomic force microscopy. Force pulling measurements were used to establish the molecular weight and the grafted chain density. The swelling transition was found to be at pH 9; which is significantly different to the pK(a) (5.5) of untethered poly(methacrylic acid). We attribute this large shift in pK(a) to the high grafting density of these brushes. This can be explained as a result of the Coulombic repulsion of neighbouring charges. High salt concentrations (0.3 M Na+) also collapse the brush layer. Conversely low salt concentrations cause an increase in the thickness of the brush, a behaviour expected for osmotic brushes. (C) 2008 Elsevier Ltd. All rights reserved.