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Aqueous E-pH Diagram is an essential tool for analyzing hydrometaUurgical and corrosion processes. Due to the requirements for environmental protection and energy saving in recent years, waste water processing and hydrometallurgical process of concentrate have been greatly developed. The construction of E-pH diagrams has turned to multi-component systems. However, there are some limits in plotting such diagrams. There is only one diagram for one multi-component system, which can not reflect the troth of the aqueous reaction. In the paper, a new computation method is proposed to construct E-pH diagrams. Component activity term is used to detemfine the boundary of stable areas. For the multi-component systems, different atom ratios of elements have been taken into account. M-S-H20 system is chosen to study since it is of importance in metallurgical solution. Compared with conventional methods, the algorithm is simple and conforms to real conditions. Key word8 computation algorithm of E-pH diagrams; component activity term; M-H20 system; M-S-H20 system; atom ratios Due to the decrease of high-grade ores, low-grade ores are commonly mined and hydrometallurgical processes become more important. Besides, environment protection gives new challenges to industrial wastewater. As an important theoretical tool, E-pH diagram for simple metalwater system is extended to multi-component systems to satisfy new requirements.The potential diagram in essence is one type of thermodynamic diagram, which translates thermodynamic data to diagrams. It is very repetitive and tedious, so that the application of computer to calculating and plotting such diagrams is always a great task for research workers [1"7] .However, being restricted to traditional methods, there is still some limits in plotting multi-component diagrams.Atom ratio of elements is seldom taken into account and the first element is regarded as the main one and the others are considered as accessory ones. Wadsley Is] pointed out this problem and plotted the diagrams of Fe-S-H20 system with different atom ratios of S/Fe. But no details and algorithms for such diagrams were illustrated.In the present paper, a new method, component activity term method is proposed to plot E-pH diagram. The algorithm solved the problem of different atom ratio, so that the diagrams conform to the actual condition better.