A series of 16 aircraft soundings was made during the summers of 1966 and 1987 to measure the vertical distribution of tritium in water vapor from the surface to an altitude of 6 km. Three types of tritium distributions were found. The type I soundings showed a generally increasing tritium concentration with increasing height. Type I1 soundings were characterized by a constant tritium concentration with increasing altitude. On type I11 soundings the tritium concentration gradually decreased to approximately 2.4 k m and then abruptly increased. These soundings are discussed in terms of the origin of the air masses, evapotranspiration, and the water vapor mixing ratio.(2) the origin of the air mass with respect to the stratospheric and continental sources and the oceanic sink, (3) the degree of vertical exchange, (4) the water vapor mixing ratio (MR) of the environmental air, and (5) the amount of evapotranspiration. Since most of these factors are controlled by atmospheric circulations, the T soundings may be best explained by con-Tellus XXII (1970). 6