July--September boundary layer humidity has been reconstructed from the δ18O of cellulose in tree rings of Pinus arizonica growing at an elevation of 2300 m in the Santa Catalina Mountains north of Tucson, Arizona. The annual occurrence of morphological features in the tree rings, often called false rings, allowed accurate subdivision of the tree rings into premonsoon and monsoon growth. Highly significant correlations were found among the stable oxygen isotope time series of the wood produced during the North American Monsoon and 1953--2000 July--September average specific humidity (P < 0.0001) and relative humidity (P < 0.0001) derived from radiosonde data. The correlation coefficients were significant against data from both the Tucson surface (788 m; approximately 922 hPa) and 850 hPa pressure levels, suggesting that the δ18O time series can be interpreted as a proxy for mean seasonal boundary layer humidity. Twentieth century July--September reconstructions of specific humidity and relative humidity are presented. There are no long-term trends in the twentieth century reconstructions of boundary layer humidity at this site.