The impact of the pH of precipitation on the physicochemical properties of aluminium phosphate adjuvants was investigated by precipitating aluminium phosphate adjuvants under constant pH conditions at pH values from 3.0 to 7.5 at intervals of 0.5. The pH of precipitation did not affect the morphology, but the point of zero charge (PZC) and rate of acid neutralization varied directly with pH of precipitation. Aggregation and protein adsorption capacity exhibited a parabolic relationship to the pH of precipitation. Minimum protein adsorption and maximum aggregation were observed at pH 5.5. In contrast to adjuvants precipitated from the same reactants but under uncontrolled pH conditions, the pH of all of the adjuvants precipitated under constant pH conditions remained constant for a 3-month aging period at room temperature.