Mass transfer and/or heat transport processes in a fuel-cell stack are controlled by first modeling a selected region of the stack with at least two fluid components separated from one another by solid material. Each fluid component of the selected region is then transformed into a respective auxiliary volume in which all of the fluid components and solid material in the region are assembled additionally and whose outer cell corresponds to that of the region. Then each auxiliary volume is divided into a lattice with individual lattice elements that are linked by references so that linked or coupled lattices result. Finally the exchange between the fluids in the fuel-cell stack is implemented via the references of the respective component lattices.