The aim of this work is to investigate the thermal stability of remote phosphor plates to be used in solid-state lighting systems, for the conversion of the blue light emitted by GaN-based LEDs into white light. A preliminary thermal characterization revealed that in normal conditions of blue light irradiance the phosphor plates can reach temperature levels higher than 60°C, which can affect both performance and reliability. The results of accelerated thermal stress tests indicate that high temperature levels can trigger a relevant degradation mechanism (estimated activation energy is 1.2eV), that drastically reduces the phosphor conversion efficiency and modifies the photometric and colorimetric characteristics of the emitted white light.