Cross-linked composite PDMS coatings containing luminescent micrometersized Ce doped Y3Al5O12 garnet particles were prepared. The garnet particles were homogeneously and individually dispersed across the bulk of the cross-linked composite coatings, which is an important requirement for the potential application of these polymer-based light converter coatings. The thermal and optical degradation of the composite coatings was investigated by TGA and after exposure to high temperature (300 C) and high power visible light irradiation (150 W) for long periods of time, under air and atmospheric humidity conditions. The composite coatings were analyzed by SEM and optical microscopy after the heat and light exposure and no phase segregation, coloring or cracking was observed. Microscope-ATR-FTIR Spectroscopy confirmed the stability of the composite coatings during the exposure to high temperature and light in the visible range, but small structural changes were revealed in the cross-linked PDMS network, mainly due to the formation of additional cross-links. The cross-linked composite coatings emit white light upon irradiation with a blue light source, even after being exposed to the degradation test, attesting for their potential application in new solid state lighting devices, i.e. high power LEDs.