Flashlamp excitation of H2O vapor in the 10–80‐mTorr pressure range has produced a population inversion on the (001), g = 3, 954−945transition. This has resulted in a modulated emission at 115 cm−1. The oscillations on the amplified spontaneous‐emission signal are interpreted as quantum beats between magnetic hyperfine levels split by anI⋅Jinteraction. Analysis of the data results in a splitting of 1.4 MHz and dephasing rates of 7.8 MHz/Torr for H2O‐H2O, 0.5 MHz/Torr for He‐H2O, and 1.7 MHz/Torr for Ar‐H2O mixtures.