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Two sampling flights were made to determine the latitudinal distribution of tritium gas in the troposphere between California and Antarctica. These data, confirmed by shipborne and land station samples, were used to parameterize the interhemispheric air exchange and the atmospheric residence time of hydrogen gas. When tritium gas is assumed to be in steady state with its sources in the northern hemisphere and when a two-box model with a 0.6-year exchange time is used, an estimate of 6–10 years hydrogen residence time is obtained. Vertical sampling flights over the central United States show the tritium gas to be well mixed up to 14-km altitude, thus corroborating the long residence time estimate.