An instrument (1) for controlling the temperature of electronic components (20) under test creates a testing environment in an enclosure within which a component contacts directly the active face of a set of back-to-back Peltier cells (50). Heating and cooling of the component (20) under test is readily achieved by controlling the Peltier cells (50) and test components may be easily interchanged using a removable mounting assembly (10, 11, 12, 13). The instrument has the advantages of being small, portable, easy to operate, inexpensive to manufacture and to run, and of producing very little electrical noise.