Abstract: A batch processing system and unit for testing electrical components in a sub-zero environment includes an isothermal chamber which mates easily with the test heads associated with conventional automatic testing equipment and which eliminates thermal gradients and variable test conditions as well as eliminating frost contamination of test heads, costly fixturing for the various types of semiconductor chip packages or passive components and static charge. The utilization of a single isothermal soak chamber provides that all devices under test and a test socket mounted on a card secured to a fixture at the bottom of the chamber are cooled to the same temperature by moisture-free gas introduced into the chamber. It is a feature of the subject invention that the test head, located outside the chamber, is well insulated from the chamber. The chamber, with all internal parts and devices, is cooled down until all parts are at the same temperature.