Abstract: A method of operating a fuel cell (1) in which a gaseous supply stream (5) comprising a reactive species is delivered to an electrode (2) where the reactive species is consumed in an electrochemical reaction to produce an exhaust stream (7, 8) which is depleted in reactive species, which method comprises: a) assessing the concentration of reactive species in the exhaust stream (7, 8); b) relating the concentration of reactive species in the exhaust stream (7, 9) to a maximum current that may be drawn from the fuel cell (1) without redox damage of the electrode; and c) adjusting the way in which the fuel cell (1) is operated in order to optimize efficiency without redox damage of the electrode (2).