TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Coolant: The coolant comprises hydrofluoroether(s), hydrofluorocarbon(s), or an azeotrope or azeotrope-like mixture of hydrofluoroether and/or hydrofluorocarbon. The coolant is preferably HFE-7100, HFE-7200, HFE-7500 and/or Vertrel HFC. Preferred Hydrofluoroether: 48 examples for hydrofluoroether, such as HFE-7500 (ethyl perfluoroheptyl ether), Freon E-2 (1-(1-difluoro(1,2,2,2-tetrafluoroethoxy)methyl)-1,2,2,2-tetrafluoroethoxy-1,1,2,2,3,3,3-heptafluoropropane), 1-(difluoromethoxy)-1,2,2-trifluoroethane, and pentafluoro(2,2,2-trifluroethoxy)-ethane, are claimed. Preferred Hydrofluorocarbon: 26 examples for hydrofluorocarbon, such as Zonyl PFBE (3,3,4,4,5,5,6,6,6-nonafluoro-1-hexene), Vertrel XF (1,1,1,2,2,3,4,5,5,5-decafluoro pentane), and trans-1,1,2,2,3,4-hexafluorocyclobutane, are claimed. Preferred Mixture: 39 examples for the azeotrope or azeotrope-like mixture of hydrofluorocarbon and/or hydrofluoroether, such as mixture of 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane (46.8 wt.%) and 1,1,1,2,2,3,3,4,5,6,6,7,7,7-tetradecafluoro heptane (6314mcee) (53.2 wt.%), and mixture of 1,1,2,2,3,3,4,4,5,5,6,6-decafluoro hexane (47.9 wt.%) and 6314mcee (52.1 wt.%), are claimed.    TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Properties: The coolant has electrical resistivity of greater than 10 to the power 7 ohm-cm, preferably greater than 10 to the power 8 ohm-cm, and most preferably greater than 10 to the power 10 ohm-cm. The boiling point of the liquid is 1-5 degreesC less than the reactor temperature at the coolant pressure. The reactor temperature is 50-95 degreesC, preferably 20-200 degreesC, more preferably 95-200 degreesC and most preferably 95-125 degreesC. The coolant pressure is ranging between slight vacuums to 60 psig, preferably less than 15 psig. The viscosity of the coolant at -40 degreesC is less than 5 cPs, preferably less than 2 cPs and more preferably less than 1 cPs, and at 80 degreesC is less than 1 cPs, preferably less than 0.5 cPs and more preferably less than 3 cPs. The coolant has freezing point of less than -40 degreesC, preferably less than -50 degreesC, at ambient pressure.Preferred Process: Preferably, all of the coolant is vaporized.Preferred Reactor: The electrochemical reactor is preferably an electrochemical fuel cell, such as proton exchange membrane fuel cell.Preferred System: The cooling system further has a pump (12) for circulating the coolant, and a heat exchanger/condenser (18) for cooling and condensing the coolant after absorbing heat from reactor.