In this book a presentation of a phenomenological theory of elec­ trochemistry is given. More precisely, it should be stated that only one part of the whole field of electrochemistry is developed. It is the purpose of this treatment to describe the interconnection between the electric current in a composite thermodynamic system and the rate of production of a certain substance on the one side, the rate of deple­ tion of another substance on the other side, and the work per unit time which has to be delivered to or is supplied by the system. The last part of this programme leads to the computation of