New rules for the exergo-economic optimization methodology for multiproduct systems are presented. The rules include the loss of exergy unit cost due to irreversibilities when it is used in a subsystem. A comparison between the methodology using the new rules and with a different set of rules is presented here for the case of a cogeneration plant similar to the CGAM problem but using a combined cycle more detailed than CGAM. We found an exergy unit value lower than that found by Valero's methodology for a waste stream. (C) 2003 Elsevier Science Ltd. All rights reserved.