Continuous gas-phase Friedel-Crafts isopropylation of toluene and cumene has been investigated using acidic Supported Ionic Liquid Phase (SILP) materials as the catalyst. Detailed kinetic studies in a multi-stage tubular reactor revealed that reactant's water content and the aromatic-to-alkene ratio are the most relevant influencing factors for the catalyst stability and selectivity in these reactions. Based on this insight, our process optimization led to the first highly stable, highly active and selective SILP-type acidic catalyst system for gas-phase aromatic alkylation. We anticipate that this concept offers great potential for more sustainable Friedel-Crafts alkylation processes. (C) 2009 Elsevier B.V. All rights reserved.