The equilibria of mildly saponified W. hemlock wood meal with aqueous sodium hydroxide were studied in the concentration range 0.5 to 5.0 N, using added sodium chloride to characterize the Donnan effects involved in the equilibria. With increasing alkali concentration, the alkali reversibly consumed by the wood meal increases, reaching the value of 1.7 eq/kg dry wood. Quantitative determinations of the accompanying increase in anionic groups associated with the wood matrix suggest that these groups are formed predominantly through the ionization of monomeric units in polysaccharides, behaving as very weak monobasic acids. Comparison of results obtained for mercerized and unmercerized wood indicate that 63 % of the polysaccharides in the latter case are accessible to equilibration at alkali concentrations of 2 N and below. Elimination of Donnan effects by sufficiently high added sodium salt concentrations can be used to determine the average acidic dissociation constant of polysaccharides in mercerized wood. The value found for ambient temperature, 0.93 · 10−14, declines slowly with increasing temperature. Water associated with the inner solution was found to increase from 0.6 to 1.3 g/g wood with increasing hydroxyl ion concentration from 0.5 to 2.0 N. In the same alkalinity range, the ratio of inner to outer hydroxyl ion concentration was shown to increase from 0.55 to 0.70 when the measurements were carried out in the presence of 0.5 M NaCl. The importance of polysaccharide ionization and accompanying Donnan effects in the interpretation of kinetic kraft pulping studies is demonstrated.