An interesting phenomenon has been discovered in the alkylation of isobutane with butene in the presence of an HPW/SiO2 solid acid catalyst under supercritical conditions. The selectivity to high octane trimethylpentane (TMP) products is increased significantly during the isobutane/butene alkylation when the trace amounts of promoter HF are added to alkylation feed. Such special action of the trace amounts of promoter in the solid acid alkylation is called the "stimulation of reaction". The kind of stimulation that occurs in the solid acid alkylation has been confirmed repeatedly in our laboratory. This is a novel experimental phenomenon in the isobutane/butene alkylation over the solid acid catalysts. (C) 2004 Elsevier B.V. All rights reserved.