A thermal instrumentation assembly which uses a high‐temperature tubular reactor for conducting gas‐phase thermal decomposition studies of organic substances has been designed. This thermal reactor assembly uses a counterflow heat exchanger in conjunction with a narrow‐bore quartz tubular reactor to obtain precise control over physical factors, such as exposure temperature, mean residence time, and gas‐phase residence‐time distribution. By virtue of the design of this quartz‐ware assembly, gaseous species are subjected to essentially a square‐wave thermal pulse as they pass through the thermal reactor.