In a process for producing a hydrocarbon composition, a feed comprising at least one C.sub.3 to C.sub.8 olefin and an olefinic recycle stream rich in C.sub.9- hydrocarbons is contacted with a crystalline molecular sieve catalyst having an average crystal size no greater than 0.05 micron and an alpha value between about 100 and about 600 in at least one reaction zone under olefin oligomerization conditions including an inlet temperature between about 150.degree. C. and about 350.degree. C., a pressure of at least 2,860 kPa and a recycle to feed weight ratio of about 0.1 to about 3.0. The contacting produces an oligomerization effluent stream, which is separated into at least a hydrocarbon product stream rich in C.sub.9+ hydrocarbons and the olefinic recycle stream.