NOVELTY - A reactant flow field comprises a first reactant channel (30) for receiving a reactant; a second reactant channel (32) for conveying the reactant away from the flow field; flow field chambers (34); and a flow transition (38) extending between the two channels for directing flow of the reactant between the first channel, the flow field chambers and the second channel. The reactant traverses the flow transition at least twice before reaching the second reactant channel.    USE - Reactant flow field for supplying reactant fuel (e.g. hydrogen) to anode, and reactant oxidant (e.g. oxygen or air) to cathode.    ADVANTAGE - The reactant flow field optimizes reactant flow to electrodes in a fuel cell, such as a proton-exchange membrane fuel cell.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a fuel cell comprising a membrane electrode assembly (MEA) having an anode side and a cathode side; a first gas diffusion layer adjacent the cathode side of the MEA; a first flow field plate contacting the first gas diffusion layer and comprising a reactant inlet (26) and a reactant outlet (28); and flow field chambers, where a reactant flows from the reactant inlet to the reactant outlet via the first gas diffusion layer and the flow field chambers;    (2) a method for flowing reactant through a fuel cell, comprising delivering the reactant to a first flow field channel; flowing the reactant from the flow field channel to flow field chamber(s); and flowing the reactant from the flow field chamber to a second flow field channel, where the reactant traverses a gas diffusion layer at least twice as the reactant flows from the first flow field channel to the second flow field channel; and    (3) a method of operating a fuel cell comprising delivering a reactant to a first flow field channel; and flowing the reactant from the first flow field channel to a second flow field channel via an array of flow field chambers and a gas diffusion layer, where the reactant flows through the gas diffusion layer at least twice.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional side elevation view of a transition flow field plate.    Inlet (26)    Outlet (28)    Reactant channels (30, 32)    Flow field chambers (34)    Flow transition (38)