Five commercially available impregnating agents were applied to Whatman filter paper from aqueous solutions of 0.1% and 1.0% concentrations by weight. Samples were dry oven-aged at 100°C for up to 14 days. Weight change, folding endurance, objective color, surface pH of the system, and reversibility to water and toluene of the impregnating agents applied to glass slides were observed. At 0. 1% concentrations, none of the impregnating agents imparted substantial strength to the system, compared with untreated paper. At 1.0% concentrations, Kelgin, Methocel, and Viscarin (but not Elvanol or Klucel-G) imparted substantial strength to the system, compared with untreated paper. After 14 days of dry oven-aging, Methocel 1.0% appeared to be best and Viscarin 1.0% worst (folding endurance below that of untreated paper). Color changes were small for all treatments, as were pH changes. All impregnating agents were completely soluble in water and partially soluble in toluene after aging on glass slides for 16 hours at 120°C.