Olefins are dimerized in a loop reactor with flashing of the reactor effluent in a flashing zone within the loop. A subsequent flash in a second flashing zone can also be used for the removal of product as flash vapor. In another embodiment, separated product dimers or product heavies are used as absorbents for unreacted ethylene. The use of a vapor-liquid contacting device incorporated in the loop reactor is especially helpful in the dimerization of an olefin when the olefin is available in low-concentration gas streams. A thermosiphon loop reactor can also be used for low-concentration olefin streams to minimize the power requirements as the energy of the feed gas is used to induce the reactor circulation.