This paper presents a method for calculating the absolute exergy of multicomponent liquid, vapour or two-phase flows. The method is different from those described by most authors in that it enables a clear division of the total exergy of a material stream into three terms such that the exergy change of mixing is calculated separately from the chemical and the physical exergy. The method has been implemented in a set of external subroutines which have been integrated with a flowsheeting simulator, in order to calculate exergies of material streams along with the traditional energy- and mass-balance calculations. Except for an external database containing values of standard chemical exergies, the exergy calculations require no additional input data. Exergies are calculated as an extensive stream property and are therefore accessible to various procedures supported by the flowsheeting simulator, e.g. optimization. Furthermore, this also enables the determination of exergy losses by simple accounting. A complete exergy analysis, of course, must also incorporate the interpretation of these exergy losses and a suggestion or indication of ways to reduce them. These will be discussed in part 2 of this article with the case of synthesis gas production from natural gas, showing the exergy method as a useful diagnostic tool for analyzing chemical processes.