The aim of the present work is to predict theoretically the nature of the mechanism of catalytic behaviour, pseudoliquid or surface-type, of Wells-Dawson solid acid (WDA). For this purpose, we took into account the dipolar moment of the substrates and the presence of H5O2+ species in their structure, considering both theoretical and experimental results. In the present paper we studied experimentally the deprotection reaction of aldehydes from 1.1-diacetates. These theoretical and experimental results together with our previous works indicate that the catalytic behaviour of WDA is a function of the dipolar moment of organic molecules and the presence of H5O2+ species. The pseudoliquid or surface-type behaviour depends on the physicochemical properties of the organic structure and its possibility of interaction with H5O2+. (C) 2010 Elsevier B.V. All rights reserved.