Active oxygen species (AOS) generated in the atmosphere can be applied in various industrial processes owing to their extremely strong oxidative ability. Methylene blue (MB) decolors upon exposure to AOS owing to dye degradation; this property can be used to detect the AOS. To detect AOS with higher oxidative ability, it is necessary to stabilize MB by mixing it with sodium alginate. In our previous work, we showed that the OH center dot concentration in the AOS increased under the high-humidity condition. Herein, the decolorization mechanism of MB-dyed sodium alginate thin films upon exposure to AOS was elucidated under low- and high-humidity conditions; decolorization was observed only under the latter. We analyzed an MB-dyed sodium alginate thin film indicator to elucidate the chemical reactions occurring as well as the decolorization mechanism generated under the high-humidity condition. We found that the decolorization of the film was caused by MB decomposition upon exposure to the AOS generated under the high-humidity condition. (C) 2020 The Japan Society of Applied Physics