A new reactor for studying the purely homogeneous thermal decomposition of organometallic precursors used in the Metalorganic Vapor Phase Epitaxy (MOVPE) of semiconductors is presented. The idea is based on the use of a counterflow jet configuration with one jet being heated and the other unheated. The heated jet contains pure carrier gas (typically hydrogen or nitrogen), while the unheated jet contains vapors of an organometallic species diluted in the same carrier gas. Under appropriate operating conditions, decomposition of the organometallic species takes place near the stagnation plane where the hot jet collides with the cool jet. Since the reactions occur in the gas phase and away from hot walls, purely homogeneous kinetics can be obtained. Such a counterflow jet reactor was designed for studying the thermal decomposition of tertiary-butyl-arsine (TBA), t-C4H9AsH2, a very promising precursor for MOVPE of GaAs films. Two-dimensional finite element simulations of transport phenomena and kinetics have been used to identify optimal operating conditions. An experimental system was constructed and capillary-sampled mass spectroscopy at the stagnation plane was used to study the thermal decomposition of TBA in nitrogen at a total pressure of 252 Torr. Gas-chromatography of the effluent gas stream was employed for positive identification of the hydrocarbon byproducts. The results indicate the existence of two major decomposition routes: (1) A low activation energy pathway producing isobutane AsH, and (2) a higher activation energy, β-hydride elimination pathway producing isobutene and arsine.