An oligomerization process in which hydrocarbon feedstocks are contacted with a hydrotreating catalyst in the absence of hydrogen and in the liquid phase. The catalyst is a heterogeneous catalyst selected from supported reduced metals, metals oxides, metal sulfides and combinations thereof. Preferred catalysts include mixed nickel and molybdenum oxides or mixed cobalt and molybdenum oxides. The process also oligomerizes sulfur compounds so that sulfur containing feedstocks can be treated without deactivating the catalysts.