In the present study the stability of the phases pertaining to the Cu–Fe–S–O–H compositional system is investigated using the Eh-pH prevalence diagrams. Calculations were performed under the PHREEQC formalism, simultaneously accounting for all concurrent equilibria. Accordingly, point-to-point mass balance diagrams were realised, changing initial parameters to explore the dependence of the system on the temperature, and on the metal-to-sulfur ratio. Among the most relevant results, discussed in comparison with the previous existing literature mainly driven by calculation performed under the so-called line method approximation, the occurrence of large field of stability of the ternary species (and, among them, of delafossite, CuFeO2) and the net change of some boundaries in the stability fields of the binary species, due to competing equilibria. The results have been discussed on the light of the occurrence of these minerals in cuprous and ferrous mineralizations, and in the perspective of possible application in Corrosion Science. •New thermodynamic modelling in the Cu–Fe–S–O–H system.•Eh-pH at temperature higher than room temperature.•Revised predominance field for ternary phases in the Cu–Fe–S–O–H.•Cu–Fe–S–O–H is not just the sum of Cu–S–O–H and Fe–S–O–H.