Experiments of microwave spectroscopy of gases at low pressures are performed with some advantages using dielectric waveguide cells. The construction of an improved version of a dielectric waveguide spectrometer is described. Details on the experimental methods utilized for gas handling and for the observation of some absorption lines of HNO3, O3, and H2O in the 18–26.5‐GHz frequency band, and of the CS radical at 49 GHz are given.