Presented in this paper are the H2O data collected during the 1977 and the 1978 Gametag (Global Atmospheric Measurements Experiment of Tropospheric Aerosols and Gases) field experiments. These data primarily reflect sampling occurring during the horizontal flight legs, involving altitudes of 5--6.5 km (free troposphere) and those in the atmospheric boundary layer at altitudes of ?2 km. The latitude range covered was 70¿N to 58¿S. Approximately two thirds of the data were recorded over the open Pacific Ocean with the remaining one third being recorded over continental areas (e.g. Canada and Alaska). The H2O results show that very large variations in the level of this trace gas can occur within the free troposphere over very small geographical distances. They also indicate that over some regions of the Pacific Ocean very low levels of H2O can be found even at tropical and subtropical latitudes. The comparison of Gametag data with existing global dew point compilations suggests that some of the radiosonde middle tropospheric dew point profiles may be significantly in error, the radiosonde results being too high. Comparisons of the Gametag O3 and dew point/H2O data have shown that for the middle troposphere these two variables very frequently are negatively correlated. Comparisons between O3 and CO, when carried out within a given hemisphere, do not show a significant negative or positive correlation. On the basis of our findings, we have concluded that stratospheric/tropospheric dynamic exchange processes, directly and/or indirectly, play a controlling role in defining the tropospheric O3 source strength.