Abstract. Effect of pH on the aluminum chloride hydrolysis at low concentration was investigated in detail by electrospray ionization (ESI) mass spectrometry. In particular, formation and decomposition processes of polymeric aluminum species were discussed. When coagulant AlCl3 was diluted to normal coagulant dose (1.5×10−4 mol/L), hydrolysis occurred immediately. Monomeric and dimeric a…