As a means of studying the kinetics of rapid reactions in which a gas is evolved, a constant volume reactor has been designed. This reactor employs a static technique in which the reaction is followed by the increase in pressure as the gas is evolved. The apparatus is capable of contacting 1 ml of one reactant with 25 ml of a second reactant and obtaining effective mixing in less than 10 milliseconds.The pressure‐sensitive device is a strain gauge, the output of which is plotted as a function of time by a cathode‐ray oscilloscope. The apparatus was tested for complete mixing and desaturation of the gas from the liquid phase by means of a reaction known to be very fast. The reaction chosen was the homogeneous reaction between the permanganate ion and hydrogen peroxide in acid solution.The reaction between lithium borohydride and sulfuric acid was found to have a half‐time of approximately one second. The half‐time of the reaction between sodium‐potassium alloy and ethanol was only a few milliseconds. The reactions of aluminum borohydride and ethylaluminum sesquihydride with water and with acids also were studied.