Humic acid was coagulated by aluminum chloride and pokyaluminum (PACl) at different pH conditions. Aluminum speciation was investigated in details through combining electrospray ionization mass spectrometer (ESI-MS) with solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). Freeze-dried flocs from coagulation precipitates were analyzed by Fourier transform infrared (FTIR) and solid-state 13C cross-polarization magic angle spinning (CPMAS) NMR. This study also evaluated removal of disinfection byproduct (DBP) precursors by PACI and AlCl3 coagulation during coagulation process. Particular emphasis was paid to the role of aluminum speciation and structure characterization in the preferential removal of precursors by coagulation. The results indicated that prehydrolyzed or in situ formed Al13 species during coagulation process exhibited preferential removal for corresponding DBP precursors with relative structures. On one hand, for DBP precursors with aromatic and carboxylic structures, in situ Al13 species were preferentially bound to these DBP precursors. Thus, more DBP precursors with aromatic and carboxylic structures were removed by AlCl3 than those by PACl at pH 5.0. On the other hand, for DBP precursors with aliphatic structures, aluminum coagulation was not responsible for reducing these precursors. However, oligomeric aluminum species and preformed Al13 species were weak preferentially bound to DBP precursors with aliphatic structures.