The combined distillation unit is the first plant in a crude oil refinery, in which several intermediate products, i.e. liquid petroleum gas (LPG), naphtha, jet fuel, kerosene, diesel fuel, gas oils and reduced crude, are obtained through atmospheric and vacuum distillation of the crude oil mixture. It is important to determine the critical points in the unit from the exergy view point and to properly allocate the total production cost to the different simultaneously produced streams, to determine the monetary flows all through the plant, and to state the relevance in economic terms of the exergy losses of each individual equipment item. The results of the exergy and exergoeconomic analyses are presented in this paper. The most important factor affecting the transformation, operation and production costs of the products is the cost of the crude oil raw material; utilities, salaries, maintenance and even capital investment costs are less important. Also the critical points of the plant are presented which provide the basis for the optimization of the unit showing additional opportunities for process integration. (C) 2004 Elsevier Ltd. All rights reserved.