A fuel cell system having a fuel cell stack (9) employing a phosphoric acid or other electrolyte, includes a non-reactive zone (11) in each of a group of fuel cells between corresponding coolant plates (55), the coolant entering the coolant plates in an area adjacent to the non-reactive zones (29 31). Each fuel cell has three-pass fuel flow fields, the first pass substantially adjacent to a third zone (13), remote from the first zone, the second pass substantially adjacent to a second zone contiguous with the first zone, the coolant flowing (33, 34) from a coolant inlet (29) through the first zone, to the far side of the second zone, and (37 41) from the near side of the second zone to the third zone and thence (45 50) to a coolant exit manifold (30), which assures temperatures above 150.degree. C. (300.degree. F.), to mitigate CO poisoning of the anode within the reactive zones, and assuring temperatures below 140.degree. C. (280.degree. F.) to promote sufficient condensation of evaporated electrolyte in the non-reactive zones, so as to provide a long system life.