A continuous catalytic process for oligomerizing lower olefins to heavier hydrocarbons wherein a continuous liquid olefinic feedstream is diluted with a liquid alkane stream, is improved by a technique which comprises cooling catalytically converted effluent, fractionating the effluent to obtain a condensed lower aliphatic recycle stream, a liquid C.sub.3 -C.sub.4 product stream, a liquid product stream consisting essentially of C.sub.5.sup.+ hydrocarbons and a gaseous C.sub.2.sup.- gaseous stream by the sequential fractionation steps of: debutanizing the cooled effluent to obtain the liquid C.sub.5.sup.+ hydrocarbon stream and a condensed lower aliphatic stream; de-ethanizing a portion of the lower aliphatic stream to recover gaseous offgas stream and a C.sub.3 -C.sub.4 alkane product stream; and optionally recycling at least a portion of the condensed aliphatic stream from the debutanizing step for dilution of the olefinic feedstream.