The depolymerization of cotton yarn irradiated with the quartz mercury-vapour lamp was found to be inhibited by both iron and copper, nominally present as hydroxides. On irradiation behind Pyrex glass, which absorbs light of wave-length less than 3000Ã…, iron again inhibited the reaction, but copper showed a slight positive catalytic effect. In the presence of small amounts of alkali, copper-impregnated yarn degraded extremely rapidly, but the process was scarecly affected by light and appears due simply to alkaline autoxidation. In preliminary experiments, it was found that (1) iron hydroxide can be precipitated in cellulose in a finely divided form, whereas copper hydroxide apparently gives only a coarse opaque deposit; and (2), of fourteen metallic oxides irradiated in aqueous suspension by ultra-violet light, only zinc oxide gave rise to the presence of hydrogen peroxide in the water.