The possibility was studied of using decene-1 as comonomer with ethylene in a slurry type polymerization with Ziegler-Natta catalysts. Under the reaction conditions used decene contents remained at the 2 wt% level in ethylene/decene-1 copolymers. When additionally butene-1 was present in the polymerization, decene-1 contents were significantly higher. A synergistic effect was identified in the reactivities of butene-1 and decene-1 in terpolymerization with ethylene. Decene-1 content had an effect on the polymer density and crystallinity, but virtually no effect on melting temperature.