The purpose of this paper is the study of a real cogeneration system existing in a petrochemical complex. The total capacity of the plant is 200 MW of electrical power and 2100 t/h of steam for processes. This work presents the mass and energy balances of this cogeneration system. Exergy of each involved stream is calculated and the exergetic balance of each subsystem is presented, as well as of the global system, identifying where and why losses and irreversibility occur. Efficiencies based on the second law of thermodynamics are calculated for each subsystem and compared. Some conclusions regarding operational strategiesare presented. (C) 1998 Elsevier Science Ltd. All rights reserved.3916-1818451852