A comparative study has been made of the kinetics of release into water of simple hydrophilic, but sparingly soluble, solutes (exemplified by CaSO 4 or SrSO 4 ) incorporated in varying amounts in cellulosic polymer matrices of low or high water swellability. Hydrophobic cellulose acetate films (cast from an acetone dope containing a dispersion of the appropriate salt particles occupying a fractional volume 1 N ร
0.1-0.4 in the loaded hydrated matrix) were found to be particularly useful for this purpose because they could be easily hydrolyzed to cellulose, thus producing hydrophilic polymer matrices containing identical amounts and distributions of solute particles. The kinetic behavior observed exhibited the same main features as previously noted in drug release studies. Thus, a t kinetic law was obeyed in all cases (apart from a relatively short initial period), while the diffusion coefficient calculated by application of the Higuchi model tended to rise with increasing solute load. This tendency was very strong in the case of the hydrophobic weakly swollen matrix and much weaker in the case of the hydrophilic one. On the reasonable assumption that the diffusion of solute in the saltdepleted matrix (which controls the release rate) occurs via aqueous pathways, the tortuosity t of these pathways was calculated and found to attain extremely high values in the case of lightly loaded ( 1 N ร
0.1) matrices. These high t values were drastically reduced upon either (1) increase of the salt load or (2) hydrolysis to cellulose. This behavior is shown to result from the fact that at 1 N ร
0.1, the salt particles were fully coated with cellulose acetate so that water taken up to fill the space vacated by released salt is in the form of globules dispersed in a weakly hydrated polymer matrix and, hence, is ineffective in providing continuous aqueous pathways. In (1), these globules are increasingly bridged by gaps left in the original loaded matrix, as a result of incomplete coating of the solute particles with polymer. In (2), bridging is similarly effected by the formation of aqueous pathways through the polymer when its degree of hydration is sufficiently increased.