Four samples of synthetic low-substitution Al-goethites (mol% Al < 10) were incubated with 10 mM NaClO4 and 5 mM MES buffer (MES buffer (2-[N-morpholino]ethanesulfonic acid) at pH 5. It was found that MES buffer, although commonly used to control pH, profoundly affected the results of our electrophoretic mobility measurements. The presence of MES buffer caused a large decrease in EM values for unsubstituted goethite, from 5.7 +/- 0.7 to 1.8 +/- 0.4 x 10(-8) m(2) v(-1) s(-1), while increases in Al substitution in goethite led to a progressive increase in EM values. The charge reversal following addition of MES buffer to suspension containing Al-goethites was explained by specific sorption that, at pH 5, is thought to occur via cation surface complexation because of (i) sulfonate induced-dipole or (ii) N and O lone-pair dipole-dipole interactions with structural Fe. (C) 2004 Elsevier Inc. All rights reserved.