Breathing circuit cannisters containing functional CO2absorbent are critical to prevent rebreathing CO2during general anesthesia using closed or semiclosed breathing systems. Ethyl violet is the indicator dye added to Sodasorb® to indicate impending exhaustion of the absorbent. A case of CO2rebreathing due to failure of ethyl violet indicator in exhausted Sodasorb® was encountered. Laboratory investigation demonstrated that dye failure could result from photodeactivation caused by fluorescent lights. Using a fixed intensity fluorescent light source and quantitative spectrophotometric analysis, a highly significant dose-response relationship was demonstrated between duration of light exposure and the decrease in ethyl violet concentration. After 24 h of fluorescent light exposure with a received flux density of 46 nwatts/cm2at 254 nm, the concentration of functional ethyl violet remaining in pulverized Sodasorb® was 16% of the baseline value. Furthermore, using multiple light sources of various intensities, the greater the intensity of light, the more rapid the rate of decline of the ethyl violet concentration. It is recommended to minimize the problem by using ultraviolet filters and incorporating additional ethyl violet in Sodasorb®. Finally, ethyl violet undergoes temporal deactivation after a Sodasorb® container is opened, even if it is stored in the dark.