Abstract: The present invention relates to a process for (co-)polymerizing olefin(s) comprising introducing into a polymerization medium the olefin(s), a titanium-based catalyst of Ziegler-Natta type, an organometallic cocatalyst and a halogenated hydrocarbon compound in an amount effective for increasing the catalyst activity in the (co-)polymerization, the amount being such that the molar ratio of the quantity of the halogenated hydrocarbon compound to that of titanium is comprised between 0.01 and 0.1, or between 0.001 and 0.15 when the (co-)polymerization is carried out continuously. The process is particularly useful in a continuous gas-phase (co-)polymerisation of olefin(s).