Free radical species, such as H, O, OH, CH₃, and SiH₃, can be ejected efficiently to the gas phase by catalytic decomposition of material gases on heated metal surfaces. The detailed reaction mechanisms for the production of these radicals are still hard to be specified, but the mechanisms can be presumed by measuring the radical densities in the gas phase under various conditions. In this review, the results on the detection of these radical species are summarized, and the possible mechanisms taking place on the catalyst surfaces are discussed. The experimental procedures to identify the radical species are also reviewed shortly. In the production of Si atoms from SiH₄, the rate determining step changes from Si desorption to SiH₄ adsorption with the increase in the catalyst temperature. In the decomposition of H₂O, the OH desorption energy depends on the surface coverage. In O₂ systems, decomposition takes place not only on the metal surfaces, but also on the second adsorbed layers.