A temperature control apparatus for a high temperature reactor in an analytical instrument of the type in which a development reagent is added to a flowing stream as part of the detection process. The temperature control apparatus utilizes solid state heat transfer means to transfer heat between a thermoconductive block to which is mounted the reactor and an air exchange heat sink which obtains heat from or disposes heat into ambient air as required. The control apparatus regulates the temperature of the reactor at a very rapid rate in the high temperature range of approximately 90.degree. C. to 150.degree. C. without the use of a separately contained heat exchange medium. The ability of the apparatus to cool the reactor rapidly eliminates the requirement to keep reactor contents at elevated pressures to prevent vaporization of the stream being analyzed.