pH-responsive polymer brushes comprise of long polymer chains of either a weak acid or base chemically attached to a surface. Depending upon the degree of ionisation along the chain the solubility of the polymer chain in aqueous environment changes dramatically, causing the brush to swell and expand, or to collapse. We propose to measure the detailed profile of pH-responsive polymer brushes on a silicon substrate as a function of pH using neutron reflectivity whilst simultaneously, for the first time, measuring the degree of ionisation along the polymer chain using in-situ FT-IR on the same sample. This will allow us to map the density profiles (neutrons) of polyelectrolyte brushes and to accurately measure their extent of ionisation (FTIR) during pH switches.