Thermocouples (TCs) spot welded on a surface, in two-phase flow, may rewet much sooner than the surrounding surface; they even may act as promoters for rewetting. In some loss-of-flow-test experiments, such spot-welded TCs are used to measure the cladding surface temperature of the fuel rods. Tests in the controlled blowdown simulation facility Karlsruhe (COSIMA) were performed using fuel rod simulators with and without such TCs. The cladding surface temperatures measured with the COSIMA pyrometers were compared, and it was concluded that the influence of the TCs cannot be neglected.