The microwave surface resistance (Rs) of YBa2Cu3O7−x (YBCO) films sputtered simultaneously on both sides of CeO2 coated sapphire wafers of 3 in. diameter was measured using the disk resonator technique at a frequency of 1.92 GHz. By deposition of Au layers of various thicknesses on the unpatterned YBCO side of the disk resonator, we studied the effective Rs of the Au-contacted YBCO films. Although the Au layer was not directly exposed to the microwave power, it dramatically increased the effective Rs of the YBCO film. For example, Rs(77 K) = 16.5 μΩ of a 300-nm-thick YBCO film increased to 85 μΩ by the deposition of a 0.1-μm-thick Au layer. The increase of the Au thickness to 1.2 μm resulted in a further enhancement of the effective Rs up to 560 μΩ. We explain this effect in terms of the impedance transformation model. According to this model the effective Rs of the Au/YBCO bilayer decreases with increasing YBCO film thickness. However, Au layers with thicknesses above 1 μm considerably enhance the effective Rs even for thick YBCO films (700–800 nm). A higher quality of the YBCO films (in terms of shorter London penetration depths) reduces the effect of a Au layer, while a low electrical resistivity of this layer leads to a further increase of the effective Rs. © 1999 American Institute of Physics.