This invention provides a process for upgrading hydrocarbon feedstock comprising the steps of:(i) recovering a C.sub.4 -rich aliphatic stream from a catalytic cracking process;(ii contacting said C.sub.4 -rich aliphatic stream with an isomerization catalyst comprising a zeolite sorbing 30 to 55 mg n-hexane at 90.degree. C., 83 torr, and 15 to 40 mg 3-methylpentane at 90.degree. C., 90 torr, per g dry zeolite in the hydrogen form in a first reaction stage to selectively isomerize C.sub.4 n-olefins to C.sub.4 isoolefins; and(iii) contacting the product stream from said first reaction stage with a solid acid alkylation catalyst selected from the group consisting of MCM-36 and MCM-49, as described herein and zeolites having a Constraint Index of less than or equal to about 2, to produce isoparaffinic alkylate gasoline.