Design criteria are examined for an electrochemical measuring system to be used for potentiostatic transient investigation of fast electrode reactions. The importance of codesign of the cell and electronics is emphasized. The cell design is examined in detail in this paper, while that of the potentiostat is treated in a separate paper. To obtain the desired characteristics of stability, good frequency response, accuracy in potential control, and uniform current distribution on the working electrode, the following features are required: small cell resistance, small reference electrode resistance, small interelectrode stray capacitances, small working electrode area, a small but variable solution resistance between the point of potential measurement and the working electrode, and a “symmetrical” electrode arrangement. A cell meeting these requirements was built. The main novel feature of the cell is the complete elimination of the troublesome “classical Luggin capillary.”