This paper describes the dissociation of water vapor in high‐temperature, solid oxide electrolyte cells. The characteristics of these cells are related to a composition‐dependent open‐circuit voltage, which can be obtained from thermodynamic considerations, to ohmic resistance, and to mass transport overvoltages. The problem of maintaining a heat balance in a system consisting of such cells is considered in terms of the heat loss through the leads connecting the cells with the surroundings. A conclusion is that the electrical energy requirement for dissociation can be reduced substantially by series connection of individual cells within the high‐temperature region of the system, but that ohmic resistance limits the optimized current density through the electrolyte to values well below the maximum values that are feasible.