The effect of light upon rosin, and compounds of rosin formed during the rosin sizing of paper has been studied, both in air and in an atmosphere of nitrogen, a relatively inert gas. The source of light used was an inclosed carbon arc. Commercial rosin and bleached rosin as well as resinates of sodium, aluminum, and iron, and rosin size contaminated with ferrous and ferric iron were exposed to the rays of the arc in quartz tubes. The fact that changes occur in an inert atmosphere, such as nitrogen, indicates that the presence of oxygen is not necessary to the chemical alteration of rosin when exposed to the action of light. The more important conclusions of this investigation follow: Rosin, both bleached and unbleached, is sensitive to light, darkening under the action of the rays. Ferrous resinate, and rosin size contaminated with ferrous iron are also light sensitive probably because of the ease with which the ferrous iron is oxidized by light. Ferric aluminum, and sodium resinates have no effect upon rosin size in so far as light sensitivity is concerned. The yellowing of paper with age may be explained in terms of the light sensitivity of rosin and of ferrous iron.