Seven ion-exchange resin catalysts were compared in the dimerisation of isobutene with tert-butyl alcohol (TBA) as the selectivity enhancing component. Experiments were also carried out with sodium exchanged resins to study the order of the reaction and the effect of sodium components in the feed. The dimerisation of isobutene was confirmed to be a second-order reaction with respect to the amount of active sites. Ion-exchanged sodium was found to decrease the selectivity for di-isobutenes and thus behave differently than the polar components. The average acidities of the acid groups of the resins treated with various polar components and of the resins ion-exchanged with sodium were obtained by NMR measurements. The results suggested that the effect of TBA and of the ion-exchanged sodium on the acidity is similar. Their different behaviour in the dimerisation of isobutene can be explained by the participation of TBA in the chain scission and by the blocking of the sites by the ionexchanged sodium. Experiments with the different catalysts showed that the surface sulphonated catalysts are the most active in the dimerisation of isobutene. Furthermore, good selectivities for di-isobutenes are obtained with the medium crosslinked resins (12-20 DVB%). (c) 2005 Elsevier B.V. All rights reserved.