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CV-1144-O silicone thin films were irradiated in an electron cyclotron resonance oxygen plasma, which is a simulation of the low earth orbital environment. A crude equivalence between this plasma system and the low earth orbital environment was determined by measuring Kapton weight loss in the plasma and comparing to Kapton weight loss in space experiments. Changes in optical properties and erosion rates under ultraviolet light and atomic oxygen radiation were studied using in situ spectroscopic ellipsometry (SE). The erosion rate at the beginning of the plasma exposure was significantly faster than that at later stages. Approximately one third of the total silicone thickness was etched away within 1 h, which according to the equivalence experiment, corresponds to about two months in low earth orbit. The refractive index of silicone in the visible range increased during the exposure, indicating that the film was being densified. Optical constants (both before and after plasma exposure) were determined by ex situ spectroscopic ellipsometry in the ultraviolet– visible–near-infrared (0.7–8.5 eV) and IR (200–7000 cm-1) ranges. Also, SE was used to map thickness and uniformity before and after radiation. Regression fits using Lorentzian and Gaussian oscillators as parametric models for the optical constants were excellent, and the major absorption peaks in the IR region were identified. The before- and after-radiation spectra showed significant decreases in CH3-associated peaks and increases in SiOx-associated peaks.