The physicochemical properties of pectin/calcium matrix tablets were studied with an emphasis on the effect of over 50%, of calcium chloride to better understand the fundamentals of the drug-release mechanisms. The absence of the adsorption of methylene blue onto the calcium-containing pectin matrix served as evidence of the calcium-ion-induced in situ crosslinking of pectin chains. Visual and scanning electron microscopy observation showed the reinforced robustness of the pectin/calcium matrix, whereas cryo-SEM revealed the formation of a pectin aggregate network as a result of calcium-induced crosslinking. The aggregate network underwent gradual erosion to a lower density, which indicated the erosion of the matrix. However, little change in the texture of the pectin/calcium matrix was observed at earlier times, which was interpreted as the initial stages of drug-release retardation. The apparent viscosity of the pectin aqueous dispersion increased biphasically as the calcium amount increased. The initial sharp increase was interpreted as calcium-induced crosslinking, whereas the following gradual increase was ascribed to a possible salt effect. We concluded that the salt effect and calcium-induced crosslinking contributed to a series of physical changes of the pectin matrix, which correlated to the sigmoidal release of indomethacin from the pectin/calcium matrix tablet. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2418-2428, 2009