The present invention relates to a process for supply of a polymerization catalyst component to a polymerization reactor which comprises: a. Providing a first stream comprising the catalyst component in a first line, which first line is connected to and downstream of a pump outlet or of a flow control valve, b. Providing a diluent stream in a second line, c. Contacting the first stream and the diluent stream to form a mixed stream and passing the mixed stream to a polymerization reactor, characterised in that the mixing of the first stream and the diluent stream takes place by providing the first stream from the first line and the diluent stream from the second line separately to a mixing chamber which has an enlarged cross-section compared to the first and second lines.