The demineralization of animal bone is investigated. Experimental results show an increase in penetration rate of hydrochloric acid into animal bone particles, and thereby increased demineralization, at increasing temperature and concentration of HCl. Fick's law is used to calculate the effective diffusivities of HCl in animal bone from these experimental results, showing a decrease in effective diffusivity at increased HCl concentration, which proves Fick's law unsuitable for describing the process, whereas a Maxwell-Stefan-based model is used successfully. This model is able to take into account gradients in composition as well as electrical potential, and also addresses the frictional resistances between the various components in the multicomponent system. The results Of the Maxwell-Stefan model are in good agreement with the experimental data. From the Maxwell-Stefan model and the experiments, it follows that at high concentrations of HCl, the friction between the diffusing ions (H+, Cl-, H2PO4, and Ca2+) becomes more important and lowers the increase of demineralization rate caused by the increase in the concentration gradient. (C) 2004 American Institute of Chemical Engineers.