Adsorption behaviors among fumarate, maleate, and succinate were compared using the same adsorbent delta-Al2O3 and initial adsorbate concentrations. The adsorption maxima were found all near the midpoint of their respective two pK values indicating that the adsorption of HX(-) was more favorable than adsorption of H(2)X and X(2-) and the adsorption densities were significantly influenced by the electrostatic microenvironment. The adsorption maxima of maleate were about 10% higher than those of fumarate but close to or slightly less than those of succinate before saturation was approached. However, when the adsorbent surface became crowded, the saturated adsorption maxima of maleate and succinate were ca. 1/3 and 1/2, respectively, higher than that of fumarate, indicating that the trans-form isomer occupied more binding space than the cis-form and the single bond anion (succinate) occupied the least binding space among the three four-carbon dicarboxylic acids due to its molecular flexibility.