1-Butene metathesis was performed over 8 wt% tungsten catalysts supported on silica, silica-alumina and gamma-Al2O3. Under the applied reaction conditions, 1-butene metathesis yields with a relatively large selectivity iso-butene in addition to the expected metathesis products. The isobutene selectivity is high for catalysts with a relative low activity and decreases with increasing metathesis activity. It is deduced that iso-butene formation involves surface tungsten complexes, whose formation inhibits the metathesis activity.