[en] The description and results of investigation of the mass spectrometer for studying fast gas phase processes are presented. The mass-spectrometer consists of a gas mixture preparation system, reactor, system of introducing the sample from the reactors in the ionization region, differential pumping-out system, ion source with electron shock ionization, 180-degree magnetic analyzer, ion electron optical converter registration system. The plant fast response equals to 10^-5 c, reactor permissible pressure - 1.3x10⁴ Pa, in the ionization region and analyzer - 1.3x10^-4 Pa. The distance from the intake peak to the ionization region is 20 mm, the length of the ionization region is 15 mm. The voltage of a steady magnetic field of the analyzer is 5500 Oe. The minimum and the maximum radii of ion trajectories in the analyzer are 25 and 145 mm, respectively. The accelerating voltage in the ion source is 600-2500 V. The range of masses measured simultaneously during accelerated voltage of 650 V is 20-490 amu. A special attention when making the device is paid to the problem of introducing the sample from the reactor in the ionization region without the distortion of composition. The effect of particle death of the reactor surface can be excluded, limiting the measurement time Tsub(meas.) by the diffusion time from the reactor axis to the walls Tsub(dif.)>Tsub(meas.). The gas dynamic overlap and a current flowing with a high rate have permitted to exclude the effect of death of active particles on the intake surface on their concentration in a molecular bunch