[en] In the process industry, it is often necessary to study the process parameters such as the residence time, flow rate, etc., under different operating conditions and equipment. The tracer technique represents in this respect an outstanding and even sometimes singular means of determining some of the above parameters. A method consisting of the introduction of a radioactive tracer at the input of a flow system under study and subsequently determining the distribution of activity with time at the output end is described. The form of the activity time curve depends on the parameters of the installation and the mode of operation. A study conducted at a multi-stage viscose rayon processing plant is described in detail. (auth.)