The experimental determination of muonium hot atom (Mu*) yields in gases is discussed, with reference to earlier work in the alkanes as well as new data in H 2 , D,. It is concluded that inelastic scattering dominates the energy loss process. In comparison with corresponding results for hot tritium, Mu* is found to be considerably less reactive. Although quantitative interpretation of this difference is not yet available, it is likely that muonium is a more sensitive probe of the topography of the underlying potential energy surface than hot tritium is.