A high‐temperature reactor system is developed for studying ultrafast gas‐solid reactions. Time‐resolved measurements of the reaction kinetics and changes in particle properties can be obtained with this system in the time scale 10–100 ms. The reactor system consists of an electrical furnace and ceramic reactor tubes, a pulse powder feed system, injection and collection probes, and a cascade impactor forinsituparticle size classification. The feed system injects solids as short pulses and is capable of handling powders of size 1 &mgr;m or less. Hot jet impaction technique is used for rapid heating of the powder to the reaction temperature. The injection and collection probes are equipped with optical systems that are interfaced with a microcomputer data acquisition system, perform particle injection/sampling, reaction quenching, and optical solids monitoring. The most unique feature of the reactor system is its ability, using the probe combination, to directly measure residence times as low as 10 ms for the solids sampled from the high‐temperature environment. Sample results are shown for the decomposition of limestone at 975 °C.