The ability of dextran (513 000 Da) with pendant acetaldehyde groups to increase the wet strength of filter paper was evaluated as a function of the aldehyde content, impregnation pH, and temperature. The impregnation of filter paper with dextran aldehyde increased the wet tensile strength. By contrast, neither dextran nor unhydrolyzed dextran acetal enhanced the wet paper strength. Drying of the impregnated sheets was a crucial step. Never-dried impregnated filter paper showed no improvement in tensile strength. This result contradicts the behavior of starch aldehyde, which can improve wet. web strength. Impregnation under acidic conditions gave greater wet-strength improvements because of enhanced dextran-fiber bond formation compared with that occurring at neutral or alkaline pH. This result was explained by an equilibration process at low pH that allows dextran-dextran bonds to be converted to dextran-fiber bonds. Wet-strength improvements slowly disappeared with rewetting time as a result of the hydrolysis of acetal and hemiacetal dextran-fiber bonds. The hydrolysis rates were fastest at low pH.