The Halogen Occultation Experiment (HALOE) is in orbit on NASA's Upper Atmosphere Research Satellite (UARS), and has been used to make measurements of a number of stratospheric and mesospheric constituents since October 1991. These include, amongst others, water vapour, H2O, and methane, CH4, two principal components of the total hydrogen budget of the middle atmosphere. The third main component is molecular hydrogen, H2, which is not measurable by HALOE or any other UARS sensor. By making the assumption that the total hydrogen content of the middle atmosphere is a conserved quantity, and that these three constituents dominate the budget, it is possible to infer the H2 fields in the mesosphere from the HALOE H2O and CH4 measurements. ¿ American Geophysical Union 1996