The microwave absorption between 100 and 300 Gc/s by H

O vapor and its mixtures with N

, CO

, and O

has been measured at room temperature for intermediate and atmospheric pressures. Near the absorption lines the line shape parameters are close to those measured at low pressures by Rusk. Between the lines, very large absorption coefficients were observed, particularly for self-broadening and for broadening by CO

. This increased absorption is ascribed to collision-induced polarization of the molecules involved. Empirical formulas are given for the calculation of line shapes. These formulas require the use of four line-shape parameters for each mixture--two of these are for the line centers, the other two for the wings.