Models of variable parameter flow processes are derived and their control is discussed. Using a pH reactor with varying flow rate as an example a variable transformation is introduced, which makes the model equations invariant under a new time scale. For a restricted class of flow processes classical analysis and synthesis methods can then be used to design a proper controller to guarantee the system stability and performance in general. The method is illustrated by developing a time-varying PID-controller algorithm as an example. Both simulation results and pilot-plant tests are reported, to verify the performance of the controller. (C) 2009 Elsevier Ltd. All rights reserved.