Abstract: The invention provides a process for transitioning between a Ziegler-Natta catalyst and a single site catalyst during the production of a polypropylene homo- or copolymer in a continuous multistage polymerisation reaction, wherein the process comprises the following steps, in sequential order: a) polymerising propylene and optionally comonomers in a first reactor and thereafter in a second reactor in the presence of the Ziegler-Natta catalyst; b) discontinuing the feed of the Ziegler-Natta catalyst into the first reactor; c) discontinuing the feed of an external donor to the Ziegler-Natta catalyst and reducing the feed of a Ziegler-Natta co-catalyst into the first reactor, wherein step c) takes place at least minutes, preferably 5 to 60 minutes after step b); d) reducing the solid content of the second reactor from a first operating level to a transitioning level in the range of 40 to 80%, relative to the level prior to step d); e) introducing the single site catalyst into the first reactor, wherein step e) t