A method for transitioning from a Ziegler-Natta to a Phillips catalyst system for the olefin polymerization reaction in one reactor, preferably a gas phase reactor, is described. The method comprises the steps of a) discontinuing a first olefin polymerization reaction performed in the presence of the Ziegler-Natta catalyst system; b) performing a second olefin polymerization reaction in the presence of a further catalyst system comprising catalyst components (A) and (B) producing, respectively, a first and a second polyolefin fraction, wherein the Mw of the first polyolefin fraction is less than the Mw of the second polyolefin fraction and the initial activity of catalyst component (A) exceeds the initial activity of catalyst component (B); and c) performing a third olefin polymerization reaction the presence of the Phillips catalyst system. Thanks to this method, there is no need to empty the reactor after each olefin polymerization reaction, and the transitioning time needed to attain the desired quality of the polyolefin obtained in each olefin polymerization reaction subsequent to the first one is sufficiently short to permit a quick and reliable change of production.