Abstract: A cell stack has frames having lines of four apertures (41) at each end. In the stack, the apertures form four ducts at each end of the side of the stack, with the ducts extending from end to end of the stack for electrolyte flow therethrough. The apertures in the transfer frames have no passages connected to them. The eight apertures (41) in the passage frame are surrounded in pairs by four grooves (44) and 0-rings (45), dividing them into a pair for anolyte feed, a pair for anolyte return, a pair for catholyte feed and a pair for catholyte return. The stack is divided into opposite end sections (46, 47). Only one of each pair is connected to a local feed or return flow passage, contained within the 0-rings. The other is connected in the other section. The anolyte feed and return passages (51,52,55,56) lead from their apertures to respective openings (61) from the side (4) of each passage frame to its plain face (18).