Abstract A system is described and tested which converts heat directly into electrical energy. It employs a solution electrochemical reaction with a small polarizability and a large molar entropy change Δ S . This is run in opposite directions in two cells: one at high temperature, where heat is absorbed, and one at low temperature, where heat is emitted. The difference in heat absorbed and heat emitted is available as electrical work; recirculation of the solutions between these cells gives a closed regenerative EMF system. The conversion efficiency of the system is high, varying from 50 to 75 per cent of the Carnot efficiency as the power output varies from maximum to 75 per cent of maximum. The power output depends strongly upon the reaction used. For the reaction tested here, the power output density was 6.4 W/m2 of cell area for operation between 90° and 30°C. Design factors for improving power output density and minimizing costs are discussed, and basic requirements for successful cell reactions are given. The feasibility of obtaining power output on the order of 2 × 102 W/m2 of cell area at 35 per cent conversion efficiency using 300°C input heat is discussed.