It is proposed that the acidic groups within the fiber wall increase the swelling of pulp in water as a result of the osmotic pressure generated by their counter-ions. These counter-ions are the trace metals normally found in pulps. Swelling is greater the lower the valency of the counter-ions (i.e., M + > M + + > M + + +) on account of the dependency of osmotic pressure on the number of ions. A decrease in pH below 7.0 causes a decrease in swelling due to the conversion of the acidic groups to their undissociated hydrogen forms. Further, by taking into account the distribution of ions between the interior of the fiber wall and the external solution, the decrease in swelling due to high ionic strength can be explained. The theory isillustrated by reference to mechanical, chemical, and chemically modified pulps. See also AATA 18-1175. -- AATA