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Unless their reliability is no longer a concern, light-emitting diodes (LEDs) will be unable to be used for broader applications. One important factor in the operating environment of LEDs is moisture, which is always present if the packaging is not hermetically sealed. In order to investigate the effects of moisture on the reliability of LEDs, several high-power white LEDs were subjected to extremely moist conditions at different temperature points in this study. Different light-output regression rates were measured. Digital microscopy was used to observe moisture diffusion on the LED module. The results demonstrate that the light output of LEDs decreases as the environmental moisture changes. Moisture diffuses into the interfaces of the packaging material, which not only decreases light output but may also disable the LED module due to electronic failure.