In this paper some theoretical and experimental results concerning the modeling and control of rectangular natural circulation loops are presented. These thermo-fluid-dynamic systems are used to refrigerate heat sources by means of natural circulation of a fluid without mechanical pumping. Despite the structural simplicity of these systems, they can exhibit unstable behaviour, resulting in wide and dangerous oscillations. A non-linear lumped parameter model was derived by using a truncated Fourier series expansion of a suitable set of variables. The performance of the model was compared with a set of experimental data. Then a set of controllers, corresponding to different equilibrium points, was designed and tested both in simulation and experimentally. In all cases the controllers were capable of stabilising the system to the desired equilibrium condition. (C) 2003 Elsevier Science Ltd. All rights reserved.