Process for introducing a polymerization catalyst in solid form into a gas-phase fluidized bed using an injection device having an inner tube of internal cross-sectional area of 10-100 mm2 and an outer tube forming an annulus around the inner tube with a cross-sectional area of 1-10 times the internal cross-sectional area of the inner tube. The polymerization catalyst and a carrier gas are passed through the inner tube into the gas-phase fluidized bed at a carrier gas linear velocity of 4-14 m/s and a carrier gas mass flow rate of 10-35 kg/h. A shielding gas is passed through the outer tube and into the gas-phase fluidized bed at a shielding gas linear velocity of 1-10 times the linear velocity of the carrier gas through the inner tube and at a shielding gas mass flow rate of 100-500 kg/h. No cooled recycle process gas is provided to the injection device.