The effect of electrode separation on the power conversion efficiency of a ferricyanide/ferrocyanide thermogalvanic cell containing platinum electrodes was studied at electrode separations up to 150 cm, using the "cold-above-hot" electrode configuration. The open-circuit potential difference is almost independent of electrode separation, but the short-circuit current density falls continuously with increasing electrode separation, while the power conversion efficiency increases asymptotically to a plateau value of ca. 0.04% at a separation near 150 cm. This corresponds to an efficiency of ca. 0.6% relative to that of a Carnot engine operating between the temperatures of 293.1 and 313.1 K used in the present study. A separation of 10 cm provides efficiencies which are ca. 83% of the plateau value and is a convenient separation to use if minimal cell volume is required. The power conversion efficiency of the cell was largely limited by ohmic overpotential. measured exchange current densities showed that activation overpotential has negligible effect on cell current. Mass transport does not limit the cell current because of the beneficial convection caused by the "cold-above-hot" electrode configuration.