In this study COOH-functionalised silica is synthesised using phosphonateN-(phosphonomethyl) iminodiacetic acid (PMIDA) in an aqueous solution. The presence of PMIDA on the silica particles was verified using Fourier Transform Infrared Spectroscopy, X-ray Photoelectron Spectroscopy and titration. Experimentally, surface concentrations of COOH functional groups of up to about 3 mmol/g(silica) were achieved, whereas theoretical calculation of the maximum COOH functional group concentration gave about 1 mmol/g(silica). The discrepancy may be caused by PMIDA multilayer formation on the particle. (C) 2011 Elsevier B.V. All rights reserved.