The swelling of cellulose gel is considered to be caused by an osmotic pressure differential resulting from a difference in concentration of mobile ions between the interior of the gel and the exterior solution. The fundamental feature within the gel, bringing about the unequal distribution, is the presence of ionizing acidic groups attached to the macromolecular network. Using the Donnan theory to describe the distribution of ions, it is shown that the experimentally observed decreases in swelling brought about by extremes of pH and by salt addition may be explained in a semiquantitative way. The swelling behavior of superabsorbent gels appears to be almost completely governed by the proposed mechanism. For conventional wood pulps, the mechanism describes the smaller variations in swelling caused by pH and salt addition. For these gels, electrolytic effects are only responsible for a fraction of the total swelling - the remainder being associated with the macromolecular network itself. The unequal distribution of hydrogen ions between a gel and an exterior solution is described briefly, along with the effect that this has on the potentiometric titration of the gel. -- AATA