We describe a compact system providing the capability of striking a sample surface with a short pulse of reactive gas in an UHV environment and monitoring the evolution of ensuing volatile reaction products. A 300-Hz rotating disk chopper synchronized with a pulsed valve produces molecular beam pulses of 12-μs FWHM at the chopper position. Only one stage of differential pumping separates the pulsed valve chamber from the target chamber with the latter under UHV conditions. The source of the molecular beam, comprised of the chambers containing the pulsed valve and the rotating disk chopper, occupies less than one square meter of floor space. Beam characterization is accomplished by the use of two UHV compatible fast ion gauges located in the target chamber. Reaction products are monitored with microsecond resolution via a differentially pumped quadrupole mass spectrometer located at right angles to the beam.