Adsorption and reaction of hydrogen (deuterium) on the Ir{111} surface has been studied with temperature-programmed desorption and direct measurements of desorbing molecules using a quadrupole mass spectrometer at similar to 100 K. H-2 exposure of the D-precovered Ir{111} surface was found to induce the desorption of HD and D-2 molecules. This result suggests that energetic H atoms (hot H atoms) produced in the dissociation process of incident H-2 molecules react with preadsorbed D atoms and desorb as HD molecules or produce secondary energetic D atoms via energy transfer. Secondary energetic D atoms (secondary hot D atoms) also induce the associative reactions with preadsorbed D atoms and desorb as D-2 molecules. We will discuss the hot-H-atom-mediated reaction based on both empirical and steady-state approximation models for interpreting the present experimental results.