A test system comprising a reactor having a fluid circuit and a test zone for an item under test. A plurality of control zones are included in the fluid circuit for controlling parameters of the fluid in accordance with control information. A control system receives input data specifying test values for the fluid parameters, predicts the behaviour of the fluid using the input data and a mathematical model of the reactor, calculates control information based on the predicted fluid behaviour, and communicates the control information to the control zones. The system can simulate transient test conditions by selective use of fluid evacuation and dilution, and by use of temperature and flow bypass circuits.