The solid-acid-catalyzed iC(4)/C-4(=) alkylation reaction has been investigated in a continuous fixed-bed reactor in supercritical isobutane and compared with that in the liquid phase. The performances of heteropolyacids were compared with those of H-beta zeolites with different Si/Al ratios. Over the two types of catalysts, enhanced catalyst lifetimes were observed when dense supercritical (SC) isobutane was used. It was observed that, when adequate solid acid catalysts are used together with convenient experimental conditions, high alkylate yields (grams of C-5(+) per gram of C-4(=)) are obtained. This demonstrates that alkylation reactions might be the operative pathways under supercritical iC(4) Conditions. Otherwise, if the cracking activity under the iC(4) SC phase is increased relative to that in the liquid phase, it is seen that, by using a convenient catalyst of lower acid strength, as shown in the present study, one can partly circumvent this drawback. With catalyst aging, the following successive steps are observed: iC(4) self-alkylation-cracking/pure alkylation-cracking/multiple alkylation-cracking.