This article focuses on mechanistic aspects of hyperthermal atomic oxygen reactions with polymers, which are the major contributor to material degradation in low Earth orbit. Due to the importance of well-controlled experiments in the understanding of the reaction mechanisms, ground-based experimental results obtained by a hyperthermal atomic oxygen beam generated by laser detonation facilities are mainly surveyed. Combined effects of atomic oxygen and vacuum ultraviolet (VUV) light on fluorinated polymers are also described. Such combined effects of hyperthermal atomic oxygen and VUV light are important not only from a fundamental point of view but also for engineering purposes (i.e., methodology for ground-based space environmental simulation). The VUV-sensitive polymers, poly(methyl methacrylate), and Teflonfluorinated ethylene-propylene do not show significant synergistic effects. Instead, the effect of combining atomic oxygen and VUV light produces erosion of the polymer that is the sum of the erosion caused by atomic oxygen and UV light acting individually. The experimental results suggest that material erosion in a complicated space environment may be quantitatively predicted if the erosion yields caused by the individual action of atomic oxygen and VUV light are known. sion energy, AO erodes many polymeric materials used in LEO. 1-3 The AO-induced erosion phenomena were recognized in early space shuttle missions. Many flight and ground-based experiments have been conducted to discover the reactivity of high-energy collisions of AO with various materials. 4-7 Since direct collisions with AO are restricted to the materials used on the exterior surface of spacecraft, AO reactions with the materials that are used in thermal control systems, solar panels, and other exposed facilities have been extensively studied. 8,9 The hydrocarbon polymer, polyimide, has been widely used in many space applications because of its superior physical and chemical properties, such as thermal stability, heat capacity, thermal conductivity, and heat resistance. It is well known that polyimide is eroded by hyperthermal AO bombardment in the LEO environment (see the Miller and Banks article in this issue). In fact, the erosion of Kapton H polyimide has been used as a standard to measure AO fluence, and such "Kapton equivalent fluence" is widely accepted for evaluating the erosion rate of other materials. 10 The erosion properties of polyimide need to be well understood, as the lack of knowledge of erosion behavior leads to uncertainty of the polyimide equivalent AO fluence.Another focus material in this article is fluorinated ethylene-propylene (FEP) copolymer. FEP also has been used as a thermal control material in many satellites. The erosion yield of FEP by AO is reported to be less than one tenth of that of polyimide in flight experiments; however, FEP erodes much faster in ground-based AO simulation experiments. 11 Due to the susceptibility of FEP to VUV degradation, it has been doubted that a strong synergistic effect of AO wi...