Two previously published surface complexation models of phenylphosphonate on aged gamma-Al2O3 and boehmite (gamma-AlOOH), respectively, have been examined by means of FTIR and FT-Raman spectroscopy. The spectral features of adsorbed phenylphosphonate were studied as a function of pH and total phenylphosphonate concentration. No evidence for a phase transformation into a three-dimensional aluminum phenylphosphonate phase was found. This suggested that phenylphosphonate is sorbed as surface complexes under the conditions used in this study. Both the infrared and the Raman data showed that the surface-bound phenylphosphonate ions undergo protonation reactions as pH is varied. These results together with the fact that the ligand has only two donor atoms for complexation and protonation led to the conclusion that a monodentate coordination to the surface is most likely in both systems. Overall, the spectroscopic results were in good qualitative agreement with the thermodynamic surface complexation models. (C) 1997 Academic Press.