Past efforts to interpret experimental data on the self‐deactivation rate of the ν2(1→0) vibrational mode of water vapor have been thwarted because of the attempt to fit the data to a single relaxation time. In a phenomenological theory proposed here the deactivation takes place by means of three parallel processes: (1) collisional de‐excitation of the excited monomer, (2) a two‐step reaction involving association and spontaneous redissociation of an H2O collision complex, and (3) spontaneous dissociation of the stably bound H2O dimer. In pure H2O and in mixtures without O2, the reaction rate for association of the collision complex is found to be very slow, and the remaining relaxation modes (1) and (3) are effectively decoupled. In mixtures of H2O–air and H2O–O2the association rate of the complex increases strongly, suggesting that O2serves as an effective ‘‘chaperon’’ for this reaction, and the relaxation modes of the monomer (1) and complex (2) become strongly coupled. Fourteen sets of past data, representing a wide variety of test conditions and experimental methods, both acoustical and nonacoustical, are organized into three groups—each corresponding to a relaxation mode predicted by the theory. An analysis of an ultrasonic absorption peak, based on process (3), yields values for the standard entropy and enthalpy of dissociation of the stably bound H2O dimer.