The influence of the physical structure of a fiber on fading by ozone of acid and disperse dyes in Nylon 6 and 66 yarns has been investigated by microspectrophotometry, a technique that permits direct observation and quantification of the distribution in dye cross-section. The findings support the hypothesis that the fading mechanism involves dye migration to the filament surface where reaction with ozone occurs. Pollutant diffusion into the filament appears to play a role in determining the ozone fading behavior. -- AATA