The diffusion theory of negative capacitance is extended to take into account potassium transport as well as proton or hydroxyl transport. It is shown that both the capacitance spectrum and the frequency at which the capacitance is zero can be used to experimentally test the theory. The effects of the fraction of potassium current, membrane conductance, NaCl concentration, and unstirred layer thickness on these two characteristics is investigated. Maximum negative capacitance can be obtained when the current flowing through the membrane is mainly carried by protons, the membrane conductance is high, the solution conductivity is low, and the unstirred layer thickness is large. The effect of a dominant hydroxyl transport in place of a proton transport is also discussed. We suggest simple experiments to test the theory on Characeaen plant cells.