A new viewpoint on metal complex system was proposed on traditional M ligand H 2O system φ pH diagrams. Simultaneous equilibrium principle and activity term method were used to plot all equilibrium φ pH diagrams for M ligand H 2O systems based on the thermochemical database developed. The mathematical and computer algorithms were described with the Cu NH 3 H 2O system as an example. More quantitative information on real hydrometallurgical processes can be obtained from these diagrams compared with the conventional Pourbaix φ pH diagram. [ A new viewpoint on metal complex system was proposed on traditional M ligand H 2O system φ pH diagrams. Simultaneous equilibrium principle and activity term method were used to plot all equilibrium φ pH diagrams for M ligand H 2O systems based on the thermochemical database developed. mathematical and computer algorithms were described with the Cu NH 3 H 2O system as an example. More quantitative information on real hydrometallurgical processes can be obtained from these diagrams compared with the conventional Pourbaix φ pH diagram. [