The effect of adding phosphate, sulfate, or nitrate anions to model aluminum-adjuvant-containing vaccines at pH 7.4 was studied. Three different aluminum phosphate adjuvants were used that had phosphate to aluminum molar ratios of 1.31, 1.18, and 0.58. One aluminum hydroxide adjuvant was used. The isoelectric points of the adjuvants were inversely related to the degree of phosphate substitution for hydroxyl. The addition of phosphate anion and to a lesser degree sulfate anion caused additional adsorption of positively charged lysozyme by a model vaccine composed of lysozyme adsorbed by aluminum phosphate adjuvant. The addition of phosphate anions and to a lesser degree sulfate anions caused desorption of negatively charged ovalbumin from a model vaccine composed of ovalbumin adsorbed by aluminum hydroxide adjuvant. Nitrate anions had no effect on either model vaccine, The effect of phosphate or sulfate anions did not change as the model lysozyme-aluminum phosphate vaccine aged, but it decreased with time in the model ovalbumin-aluminum hydroxide vaccine. The effect of phosphate or sulfate anions in producing desorption was also influenced by the amount of adsorbed protein. More desorption occurred when the ovalbumin concentration was at the plateau region. The adsorption of phosphate anions was found to lower the isoelectric point of both adjuvants. Thus, one mechanism acting to produce adsorption or desorption may be an increase in the negative surface charge of the aluminum-containing adjuvants. This in turn decreased the electrostatic interaction with negatively charged proteins and increased the electrostatic interaction with positively charged proteins.