The recombination of oxygen and nitrogen atoms on the surfaces of two coating m materials of the Space Shuttle Orbiter (SSO), a reaction cured glass (RCG) and a spinel (C742), was investigated. The recombination probability, $gamma$, i.e., the probability that atoms im p pinging on the surface will recombine, was measured in a diffusion reactor. Value of $gamma$ for oxy g gen atom on C742 ($3 { imes} 10^{-2}$) was much higher than that on RCG ($4 { imes} 10^{-4}$) at the tempera t ture of SSO re-entry (ca. 1000K). The higher value of $gamma$ on C742 indicates a higher number d density of active sites than RCG. It suggests the possibility of designing less active surfaces by i inducing the desorption at lower temperature.