Virtually, all previous heat transfer studies on chemical heat pumps have been performed using steady-state concepts, even though they are cyclical processes. In chemical heat pump reactors, one of the issues is the change in thermal inertia of the reactor contents with cycle time. In this laboratory study, the thermal inertia could be determined experimentally, as the difference between the instantaneous rate of heat generated by reaction (the change in weight of the reactor measured by a load cell multiplied by the molar heat of reaction) and the heat transferred (measured by the cooling water flowrate and its change in temperature). The standard equation for heat transfer at steady state indicates that plotting heat flux against temperature difference produces a straight line whose slope is the overall heat transfer coefficient. The slope obtained using the unsteady-state data obtained in this work varied from zero to infinity, which was an indication that the standard steady-state approach does not apply to unsteady-state processes. The unsteady-state data were expressed in terms of an instantaneous overall heat transfer coefficient that varied with cycle time and having a maximum value that was seven times larger than its minimum value. It was found to have its maximum value when the heat flux was at its minimum and a minimum value when the heat flux was at its maximum. Since the pressure and temperature were shown to be invariant with the position in the reactor (although they varied greatly with cycle time) and on the basis that the geometry of the CoCl2/fibre site at which the heat is generated is the same, it was suggested that the unsteady-state instantaneous overall heat transfer coefficients reported in this study, would also be applicable to larger reactors. Using heat transfer coefficients that vary with cycle time is definitely more representative than the use of a single value that was obtained by applying a steady-state analysis to a cyclical chemical heat pump process. Crown Copyright (C) 2004 Published by Elsevier Ltd. All rights reserved.