Since the last few years, white light emitting diodes (LEDs) are rapidly replacing both incandescent and compact fluorescent lamps for lighting: they are more compact, more energy efficient and have a much longer lifetime. White LEDs typically consist of a blue-emitting LED chip, combined with one or two phosphors which convert part of the blue light to green and red, giving an overall impression of white light. Currently, the lifetime of LEDs is not only limited by that of the LED chip itself, but also by the degradation of the phosphor materials. These powders or thin films - inorganic compounds containing a small amount of impurity elements (dopants) as light emitters - often suffer from environmental conditions (moisture and high temperature) and occasionally from the high dose of blue light from the LED chip itself. The aim of the current project is to study the degradation phenomena occurring in phosphor powders and thin films for LEDs. This will include accelerated aging experiments at high temperature, high humidity and under strong illumination in a climate chamber, but also experiments using advanced surface analysis techniques. The latter is necessary to understand the exact pathway of the degradation. Finally, powder particles will be encapsulated in order to protect them from degradation. This, together with an improved understanding of the physics of degradation in common LED phosphors, should lead to an improved performance and reliability of white LEDs.