"E"-pH (Pourbaix) diagrams provide an important graphical link between the thermodynamic calculations of potential, pH, equilibrium constant, concentration, and changes in Gibbs energy and the experimentally observed behavior of species in aqueous solutions. The utility of "E"-pH diagrams is extended with the introduction of an additional variable, "X," which is a property of one of the pertinent species such as concentration. By allowing an investigator to visualize the interdependence of three variables, these three-dimensional "E"-pH-"X" plots increase interpretive capabilities. The full usefulness of such three-dimensional diagrams is realized when the image is freely rotatable. The reader is taken through the historical development, use, and construction of two-dimensional "E"-pH diagrams and is then introduced to the methodology for producing the three-dimensional diagrams in detail for magnesium, aluminum, and iron. Supporting information is supplied such that readers can access and use the freely rotatable diagrams. (Contains 3 tables and 10 figures.)