Little work has been done on the closed loop response of refrigeration plant using the new chlorine-free refrigerants. A simulation model of a water-water refrigeration plant using R134a is developed for the purpose of investigating control system performance. The well-established problem of coupling between the two main regulatory control loops governing evaporator degree-of-superheat and plant capacity is recognised and the two control loops are tuned in harmony using an optimisation technique. To test the applicability of the control system design, the tuned controllers are applied to an identical plant but this time using the zeotrope R407C. Results show that the two control loops can be satisfactorily tuned to give minimal interaction and that the choice of working fluid has minimal effect on plant behaviour.