The work reported in the present paper is a continuation of earlier work in the photochemical decomposition of wool. The data confirm the hypothesis that the primary action of the radiation, which results in a change in the state of the sulfur and a corresponding lowering of the cystine content, is independent of the moisture in the system. The results indicate, however, that moisture plays an important role in the secondary process, namely that involving the evolution of hydrogen sulfide from the irradiated fibers. It is also shown that decomposition of the cystine resulting from thermal reactions under the conditions of these experiments in negligible.