A gas phase polymerization process comprising: (1) preparing a solution of a catalyst precursor comprising a mixture of magnesium and titanium compounds, an electron donor and a solvent; (2) adding a filler to the solution from step (1) to form a slurry; (3) spray drying the slurry from step (2) at a temperature of 100 to 140.degree. C. to form a spray dried precursor, (4) slurring the spray dried precursor from step (3) in mineral oil, (5) partially or fully pre-activating the catalyst precursor by contacting the slurry of (4) with one or more Lewis Acids, and (6) transferring the partially or fully activated precursor from step (5) into a gas phase reactor in which an olefin polymerization reaction is in progress.