In a process for oligomerizing a C.sub.2 to C.sub.6 n-olefin feedstock over surface deactivated ZSM-23, the feedstock contains from about 0.1 wt % to about 25 wt % of an iso-olefin and the C.sub.12+ fraction of the oligomerized olefin product contains less than 0.5 atom % of quaternary carbon atoms.