Off-line, in situ activity recovery of a partially deactivated USY zeolite catalyst used for isobutane/ butene alkylation was examined in a continuous-flow reaction system employing supercritical isobutane. Catalyst samples were deactivated in a controlled manner by running them to either a fixed butene conversion level of 95% or a fixed time-on-stream of 3 h and then exposing the catalyst to supercritical isobutane to restore activity. Activity recovery was determined by comparing alkylation activity before and after the regeneration step. Both single and multiple regenerations were performed. Use of a 95% butene conversion level criterion to terminate the reaction step afforded 86% activity recovery for a single regeneration and provided nine sequential reaction steps for the multiple regeneration studies. Employing a fixed, 3 h time-on-stream criterion resulted in nearly complete activity recovery for a single regeneration, and 24 reaction steps were demonstrated in sequence for the multiple regeneration process, producing only minor product yield declines per step. This resulted in a 12-fold increase in catalyst longevity versus unregenerated catalyst.