A phosphinimine catalyst immobilized on a passivated inorganic oxide support, had high activity at low co-catalyst concentrations and gave, under gas phase polymerization conditions, ethylene copolymer with a high molecular weight. A method of making a passivated silica support involves treatment of silica with an organoaluminum compound, a diorganomagnesium compound and a source of chloride (to make MgCl2) under anhydrous conditions and in the absence of polar solvents.