The degradation of polymer coating systems due to UV irradiation is studied using positron annihilation spectroscopy. Doppler broadened spectra of positron annihilation are measured as a function of slow positron implantation energy in a series of polymer coating systems which were exposed to UV irradiation for up to 2000 h. The S parameters from the Doppler broadened energy spectra vs positron energy show an increase at very low positron energy (<575 eV) and then a decrease to about 8 keV, followed by a slight increase up to 50 keV. The UV irradiation systematically decreases the S parameter as a function of exposure duration. The significant S parameter decrease is interpreted as the degradation of polymers and the change of the sub-nanometer defect profiles due to UV irradiation. These interpretations are supported by the data from the atomic force microscope, UV absorption, Fourier transform infrared, and profilometry measurements.