A spectrometer, with a grating of the echelette type, was used in mapping the absorption spectrum of ammonia gas in the region from 1 to 4 cm wave-length. To accomplish this, it became necessary to produce electromagnetic waves of shorter wave-length than had been produced before by vacuum tubes. Oscillators of the magnetron type were used as the source of the continuous short wave radiation. The center of the absorption band had been predicted to be at 1.5 cm wave-length. The maximum absorption was found to occur at 1.25 cm. This shift is explained by the unsymmetrical nature of the line. The constants in a theoretical formula for the absorption coefficient have been determined. These constants involve an effective collision diameter of the molecule, and lead to the value σ=8.8×10-8 cm. The total intensity of the line was found to be 10×107 cm-1 sec.-1.