Design simulations of and experiments in a new reactor used for studying homogeneous decomposition kinetics in a wall-less environment are presented. The idea is based on the use of two counterflowing jets with one jet being heated and the other kept cool. The heated jet contains pure carrier gas while the cool one contains the reactant diluted in the carrier gas. Under appropriate operating conditions, decomposition of the reactant takes place near the stagnation point where the two jets collide. Since the reactions occur in the gas phase and away from hot walls, purely homogeneous kinetics can be obtained. Computer simulations of the transport phenomena and chemical kinetics underlying the thermal decomposition of monoethylarsine (MEA) were used to study the structure of the flow, the size, and location of the reaction zone and to identify optimal reactor shapes and operating conditions. An experimental system was constructed and used to study the thermal decomposition of MEA and tert-butylarsine (TEA), which are safer alternatives to arsine for metalorganic chemical vapor deposition (MOCVD) of compound semiconductors. The concentration of reactants and products was monitored by in situ capillary-sampled mass spectrometry coupled with gas chromatographic analysis of the total effluent stream. The reactor model was used to fit the activation energies of the overall decomposition reactions. The results demonstrate the feasibility of the proposed reactor.