An apparatus for testing bare wire and complex thermocouple probes is described which is capable of producing a gas stream which is stable and uniform over a wide range of conditions. The gas temperature is uniform within ± 1°F and stable with time to within ± .25°F at 1600°F across a 1 × 2 in. flow area. The means of removing the fluctuations and gradients are discussed. Various environmental conditions may be imposed in the test section to allow evaluation of radiation error, conduction error or response rate. Instrumentation useful in the study of thermocouple behavior is discussed.