Although the four-carbon dicarboxylic acids (fumarate, maleate, and succinate) can be adsorbed specifically on hydrous delta-Al2O3, the isoelectric point (IEP) was not observed to shift in the three adsorption systems studied due to insignificant adsorption near the IEP and low intrinsic binding affinity of these acids for the oxide surface. Evidence of low intrinsic affinity arose from the strong ionic-strength dependence of the adsorption behavior for each of the three FCDAs. The ionic-strength effects on the adsorption behavior found for maleate were close to or slightly stronger than those of succinate, while fumarate exhibited the strongest ionic-strength dependence among the three FCDAs. These results were well related to their respective adsorption maxima (under-saturation) investigated in part 1. Therefore, it is suggested that, under the same experimental conditions, the adsorption of a particular anion will be attributed to specific adsorption if the ionic-strength effects of this anion are weaker than any of the three FCDAs studied. In addition, the lack of a shift in the isoelectric point cannot be used for judging whether the adsorption is nonspecific.