Exploring the catalytic potential of organic acid functionalized mesoporous silica in condensed phase necessitates characterization of their acidic properties in a similar environment. In this study, potentiometric titration is used as a direct measurement technique to determine the acidic strength and total acid capacity of propylsulfonic, arenesulfonic, ethylphosphonic, and butylcarboxylic functionalized mesoporous silica catalysts in water. While the method can provide direct insight into the acidic properties of the materials the characterization conditions must be carefully chosen to yield reliable results. The addition of salts are required for enhancing the ion-exchange in the titration procedure, but their over-addition leads to significant deviation of the activity coefficients leading to incorrect results. Titrations, which are performed under reliable characterization conditions, demonstrate that network interactions can exist between the organic acid functional groups, but their interaction with surface silanols does not appear to play a significant role on acidic properties. While the strongest acidity is observed for the arenesulfonic acid functionalized material, none of the acidic materials are leveled in an aqueous solvent. (C) 2011 Elsevier B.V. All rights reserved.