The present invention relates to methods for preparing amorphous aluminum hydroxyphosphate. An aluminum salt and a phosphate solution are co-mixed at a constant ratio in the presence of a buffer. Preferably, an excess of the phosphate solution is used to act as a buffer. Due to the presence of a buffer, the pH is maintained constant during reaction (after initial rapid equilibration) without active adjustment. The methods are particularly applicable for the large scale manufacturing of aluminum phosphate adjuvant. Aluminum phosphate is used as an adjuvant in vaccine formulations, particularly those including a protein or saccharide antigen.