Abstract Aqueous stability diagrams are frequently used to assist in interpreting the chemical mechanism of many hydrometallurgical systems. It has been found that, for multi-element diagrams, when chemical species containing more than one element, other than oxygen or hydrogen, are stable, more than one diagram is required to describe the system. The structure of multi-element diagrams was found to be primarily dependent on restrictions placed on the relative quantities of the component elements. The restrictions are determined by the stoichiometry of stable, multi-element species. Aqueous stability diagrams, calculated by the CSIRO-Monash Thermochemistry System for the iron-sulfur-water system at 523.15 K, and including a representation of the pyrrhotite solid-solution phase, are used to illustrate these features. Use of these diagrams in the interpretation of the mechanism of hydrometallurgical processes is discussed.