Abstract: Process and apparatus for transiently measuring surface temperatures and heat flux at the interface of two media, one of which is solid. In a solid wall through which a thermal stream flows, a cut is made which opens into the interface between the wall and the other medium. The cut is parallel to the stream at a point of contact corresponding to the interface. A microthermocouple is arranged in a plane of the cut in the immediate vicinity of the interface. The cut is closely covered by the material of the solid wall or a material with thermal characteristics similar to those of the peripheral material.