The reversibility of solvent swelling and the existence of an equilibrium swelling ratio, qeq, has been established for pigment gallstone particles. A simple technique was devised to demonstrate that solvent imbibition in pigment gallstone particles is reversible. This finding confirms unequivocally that the molecular makeup of pigment gallstones involves polymer networks. In addition, this new method permits serial direct observation of the interactions between pigment gallstone particles and the solvent. Application of the technique to other network polymers should be readily possible.