(Research Paper No. 1149.) -- The effect of clay, Ti pigments, ZnS pigments and CaCO₃ (chalk) when used as fillers, and of rosin sizing, upon the stability of 72 book papers, prepared from a wide range of fibrous materials, was evaluated by means of the accelerated aging test. Rag andpurified woodpulp papers proved more stable than sulfite-soda woodpulp papers. Non-alkaline fillers had no apparent harmful effect on stability, and CaCO₃ exerted a protective or inhibiting effect against aging. Rosin likewise had practically no bearing on stability, but acidity was an important factor, the degradation of cellulose increasing in both sized and unsized papers as the amount of alum used was increased. The various nonalkaline fillers showed no marked differences on the degree of sizing and even CaCO₃-filled papers were satisfactory as to sizing. Strength of all experimental papers decreased with increasing filler content, irrespec