The nature of the sites on silica-alumina for propylene polymerization has been investigated using adsorption, deuterium tracer, and ammonia blocking techniques. Adsorption studies indicate that propylene, like 1-butene, rapidly undergoes polymeric complex formation which ceases after a few minutes. The polymeric complex remains adsorbed on catalyst sites. 1-Butene readily isomerizes over the propylene complex. Deuterium tracer studies indicate that little propylene isomerization occurs over silica-alumina at 25 °C. The ammonia blocking studies show that polymeric complex formation for propylene and 1-butene over silica-alumina is very similar. In both cases the average degree of polymerization is four. Catalyst sites of intermediate energy appear to be the most active in polymeric formation. © 1969.