There is described a method of flowing reactants through a fuel cell stack having a plurality of fuel cells, the method comprising: dividing the stack into a plurality of groups, each of the groups connected together electrically in series; selecting a number of the groups and a number of cells in each of the groups to maintain a substantially constant stoichiometry for each of the groups, wherein a number of the fuel cells in each of said groups is decreasing from upstream to downstream; and distributing the reactants in series to each of the groups and in parallel within each of the groups.