The effect of a 6.35-mm-diameter cylindrical rod on the transitional flow in a 50.8-mm square channel has been investigated experimentally. A Chromel-Constantan thermocouple with 12.7-mu m-diameter leads was used to measure the temperature fluctuations of slightly heated air (5 degrees) flowing in the channel, over the range of Reynolds number Re = 1,500-3,500. Without the rod, laminar-to-turbulent transition occurred at Re approximate to 2,500-3,500 in the empty square channel. The rod destabilized the flow and advanced the onset of fluctuations profoundly. Within the woke of the rod, large fluctuations were detected. Temperature measurements also reviewed the existence of regular vortex shedding at Re approximate to 1,500, and the development of the woke downstream of the rod. This study confirmed the feasibility of using a fine thermocouple for studying obstructed channel flow.