Papers were irradiated with a carbon arc through a filter that completely eliminated infrared, and ultraviolet shorter than approximately 330 millimicrons. Sheet temperature was kept near 30°C during irradiation, through contact with a thermostated metal backing, thereby eliminating heat effects, shown otherwise to overshadow light effects, using intense sources. In consequence, the results differed from those of previous workers. Yellowing of papers (without lignin) commonly ascribed to light was found to result from heat or age, but not light; papers bleached when heat effects were eliminated during irradiation. Even lignified paper was bleached by light in nitrogen. Paper scorched brown at high temperatures or yellowed at 100°C, and yellow papers 250 years old were bleached by light. Lack of oxygen inhibits but does not altogether prevent photochemical deterioration. The role of water vapor differs fundamentally for cotton and wood-pulp paper. Lignified paper is very unstable to light. Printers' ink extensively protects paper. Irradiated papers are subsequently less stable in the dark than those not previously irradiated. The order of photochemical stability of papers was as follows: new-rag, refined sulfite, old-rag, soda-sulfite, and newsprint. The light stability of new-rag papers was greatly affected by acid and rosin, whereas that of old-rag and soda sulfite was only slightly affected, in contrast to heat stability, for which pH is important for all fibers. Newsprint, made neutral with NaHCO₃, showed a large increase in light stability. Rosin did not seriously affect the stability of any papers as long as their acidity was low.