Using fluorescence and optical monitoring techniques, the process of coagulation of natural humic acids (HA) was studied paying attention to the interaction of aluminium and HA. Experiments were conducted using a jar tester equipped with a PDA2000 monitoring device for online monitoring of the Al-humic coagulation process. After flash mixing, slow mixing and settling, the supernatant was collected for fluorescence analysis to measure the concentration of residual aluminium which has not reacted with HA molecules. The process of aluminium hydrolysis was also studied by online monitoring of the formation of ‘alum flocs’ in a solution free from HA. Electrophoretic analysis and TOC measurement were conducted respectively for evaluating the condition of charge neutralization and HA removal. On this basis, the mechanism of Al-humic coagulation was discussed. It can be concluded that at pH 5, charge-neutralization and co-precipitation is the main mechanisms of coagulation, while at pH 7 the main hydrolyzed species are precipitated aluminum hydroxides so that sweep flocculation becomes the main mechanism of coagulation as well as overdosing at pH 5.0.