When a system produces two or more products, cumulative exergy (CEx) should be allocated to each product. Separation process is a common operating unit, which is characterized by multiple products. Based on the minimum separation work (MSW) theory, a novel allocation parameter of CEx, the minimum separation power of product (MSPP), is proposed in this article. The natural causality between the MSPP and total CEx is identified. The related mathematical models are established. Finally, application of this approach to a case study involving a typical four-product distillation is given. (c) 2006 Elsevier Ltd. All rights reserved.