This work presents a procedure for calculating the COP and heat transfer rates, based on on-site experimental temperature measurements, of a lithium-bromide/water direct-fired double-effect absorption chiller in reverse parallel flow configuration, running on natural gas. The chiller was equipped with a set of thermocouples which allowed measuring its working temperature levels through all its operating stages. The chiller analysed in this work is the central cooling system of the air-conditioning installation of the Principe Felipe Science Museum, located at the Valencia's City of Arts and Sciences (Spain). This installation is capable of providing a cooling capacity of 4.5 millions of kcal/h (5.2 MW), by means of three direct-fired double-effect absorption chillers. From the experimental measurements a calculation procedure, based on energy and mass balances, has been developed, which allows estimating the specific powers by unit of mass flow rate through the evaporator. From these power values the instantaneous COP of the chiller could be obtained. Additionally, the paper analyzes different aspects that were not possible to be considered and details the actions taken in order to take them into account. (C) 2008 Elsevier Ltd. All rights reserved.