The absorption coefficients and dielectric constants of sixteen gases have been measured at the two wave‐lengths &lgr;=1.24 cm and &lgr;=3.18 cm. The gases are H2S, COS, (CH3)2O, C2H4O, C2H5Cl, SO2, NH3, six halogenated methanes and three amines. Certain improvements in technique are described; these improvements permit detection of absorption coefficients as small as 0.2×10−4cm−1and measurement of larger coefficients with an accuracy of ± 5 percent. The measured dielectric constants at these wave‐lengths are essentially equal to the static values. A quantitative interpretation of the absorption coefficients in terms of the known structure and spectra of the individual molecules is given. The theory indicates that all non‐planar molecules which possess a permanent dipole moment should show appreciable absorption in the microwave region.