An experimental capillary pumped loop (CPL) device has been developed in order to study its fundamental response mechanisms following steps of applied power. A global model including all the loop elements and the essential physical processes has been designed, with a sole control parameter for fitting experiments and simulations. By comparing experimental results and simulations for upward and downward steps of heat load, the response mechanisms within the loop can be analyzed. This analysis has revealed a particularly good agreement for damped oscillations of low frequency observed during the undershoots; it also provides an explanation as to the two-phase loop reacts badly to abrupt decreases of applied power. (C) 2003 Elsevier Ltd. All rights reserved.