Alkenes, such as normal butenes in a mixed C.sub.4 stream are oligomerized, preferably to dimers, which are dimerized in a distillation column reactor over ZSM-57 zeolite catalyst at high conversions and high selectivity to octenes. Prior to oligomerization the mixed C.sub.4 stream is pretreated to remove dimethyl ether, butadienes and sulfur compounds.