PURPOSE: To fractionate a stream of 3C, 5C-olefinic hydrocarbons in the presence of an intermediate pore shape-selective solid catalyst under oligomerization conditions to efficiently convert the lower olefins into 5C+ gasoline.

CONSTITUTION: (A) A hydrocarbon stream contg. 3C- and/or 4C-olefinic hydrocarbon is brought into contact with (B) an intermediate pore shape-selective solid catalyst under oligomerization conditions in an oligomerization zone to obtain an effluent rich in hydrocarbons in 5C+ gasoline range. The effluent is separated to obtain a 3C- hydrocarbon stream and a 3C+ hydrocarbon stream. The 3C+ hydrocarbon stream is fractionated to obtain a hydrocarbon stream in 5C+ gasoline range, a stream rich in 4C hydrocarbons and a stream rich in 3C hydrocarbons. Thus the starting lower olefinic hydrocarbons are converted into hydrocarbons in 5C+ gasoline range.