Ohmic, or (IR)-R-2, losses occur in electrochemical devices, such as proton -exchange membrane fuel cells (PEMFCs) due to the resistance of materials and the contact resistance between the electrochemical active area, the gas diffusion and current collector materials. Such losses must be lessened to maximize the conversion of chemical energy to electricity rather than heat. We probe how to decrease the contact resistance in PEMFCs with metal bipolar plates with state-of-the-art membrane electrode assemblies, specifically Au/TiO2-coated titanium bipolar plates (BPPs), gas diffusion layers with microporous layers (GDLs with MPLs), and catalyst-coated membranes (CCMs) comprising a 15-mu m-thick proton-exchange membranes. Through in situ tests of a model system, we find that the system resistance decreases after a compression break-in cycle and then remains low subsequently by keeping the stack assembly compressed at >1 MPa. Ex situ tests show that the surface roughness of the BPPs also affects contact resistance with the GDLs. After accounting for the bulk resistance of the cell constituents (BPPs, GDLs, MPLs and CCMs), we conclude that in a state-of-the-art PEMFC, the contact resistances between the materials contribute 55% of the total (IR)-R-2 losses, and thus dominate the ohmic loss contributions. Published by Elsevier B.V.