A supported catalyst for olefin polymerization comprises a selected ionic activator, a selected organometallic catalyst and a support material. The selected activator must contain an active hydrogen moeity. The organometallic catalyst is characterized by having a phosphinimine ligand and a substituted cyclopentadienyl ligand (which contains from 7 to 30 carbon atoms). The supported catalyst exhibits excellent activity in gas phase olefin polymerizations and may be used under substantially “non-fouling” polymerization conditions.