The optical degradation of polydimethylsiloxane (PDMS) was investigated under exposure of 150 keV protons to reveal the damage behavior of organic silicone in space. The exposure was performed in a ground-based simulation for space protons. The experimental results show that the optical degradation mainly occurs in the visible region. With increasing proton fluence, the absorption band shifts toward longer wavelengths (red-shift) and the solar absorptance a, ascends. The analysis of Fourier transform infrared spectroscopy indicates that under the exposure, the degradation of the PDMS occurs to form hexamethylcyclotrisiloxane, the amount of which increases with increasing the proton fluence. This could cause an increase in delocalization degree of the sigma electrons, resulting in the red-shift of absorption bands. Also, the energy for valence electrons to be excited decreases and the n ->pi* jump is intensified, increasing the probability of optical absorption. As a result, the solar absorptance a, of the PDMS increases under exposure of 150 keV protons. (c) 2008 Wiley Periodicals, Inc.