<b>Abstract</b>— <p>Polymer coatings are applied onto the surfaces of low-orbit spacecraft, where they are exposed to the aggressive action of incident flows of atomic oxygen. During prolonged orbital operation (10–20 years), the fluence of atomic oxygen reaches 10²² cm^–2 and more; this leads to destruction of the polymer surface to a depth of several hundred micrometers. Three types of promising coatings based on organosilicon polymers are studied: the ECT-PC composition, ECT varnish, and UF-7-21 sealant, which are expected to be used on low-orbit spacecraft. To assess their resistance to atomic oxygen, when simulating an incident flow with a high fluence of up to 10²² cm^–2 under laboratory conditions, a technique for accelerated tests in an oxygen plasma flow with an oxygen particle energy of 10–40 eV is applied. The dependences of mass loss on the equivalent fluence are studied; the measured erosion coefficients of the coatings of the ECT-PC composition, ECT varnish, and UF-7-21 sealant are found to be 4.2 × 10^–26, 3.2 × 10^–26, and 1.7 × 10^–26 g/O atom, respectively. In comparison with polymers used in spacecraft (for example, polyimide with an erosion coefficient of 4.3 × 10^–24 g/O atom), the measured erosion coefficients are two orders of magnitude lower, which characterizes the high resistance of the studied materials to atomic oxygen. On the basis of the obtained dependences of sample mass loss on plasma fluence, the predicted limiting fluence of atomic oxygen is (7–25) × 10²³ cm^–2 depending on the type and thickness of the coating.</p>