PURPOSE: To obtain high quality naphthene suitable for producing alcohols for plasticizers from a 4C fraction prepared by thermal or catalytic cracking of heavy or light oils in good yield by combination of a polynaphtha process with a dimerization process under optimum operating conditions.

CONSTITUTION: A 4C fraction obtained by thermal or catalytic cracking of heavy or light oils is subjected to polynaphtha-process cracking treatment in the presence of a silica-alumina catalyst under conditions of 15-200°C reaction temperature, 0.1-1.5l/hr liquid space velocity and 25-98% butene reaction rate and the resultant process oil is subsequently distilled at 30-230°C temperature to separate a butene-butane fraction, which is then subjected to dimerization process treatment in the presence of the above-mentioned catalyst or an alkyl- aluminum catalyst under conditions of the aforementioned reaction temperature, above-mentioned liquid space velocity and 50-90% octene conversion rate and further fractionally distilled. Thereby, an octene mixture which is a fraction of 80-130°C is obtained to suppress residual isobutylene in the polynaphtha process and improve both dimerization and octene yields.