This study investigates the mechanism of Dissolved Organic Carbon (DOC) removal from water with high alkalinity and DOC, typical in the Canadian Prairie, by three aluminum based coagulants: aluminum sulphate (alum), polyaluminum chloride (PACl), and aluminum chlorohydrate (ACH). Our focus is to discern the role of aluminum species: Al-a, Al-b, and Al-c to explain the performance of these coagulants in the removal of DOC. Removal of organic compounds is quantified by measurement of DOC, DOC fractions, and UV254. Results show that coagulation with alum at pH of 6.0 achieves highest DOC removal attributed to the highest content of in situ formed polymeric species (Al-b). At pH adjusted to 7 and 8 ACH shows the highest content of Al-b and consequently better removal of DOC compared to alum and PACl. When no pH adjustment is applied, coagulation with ACH achieves the highest DOC and UV removal, because of the highest concentration of Al-b and Al-c species in the solution. Trihalomethane Formation Potential (THMFP) of the water after the application of coagulation has also been studied. Water coagulated with alum shows the lowest trihalomethane formation potential (94.7 mu g L-1 T) in comparison to the raw water (202.4 mu g L-1) followed by ACH and PACl. This can be related to the coagulant effectiveness in reduction of hydrophobic acid (HPOA) as the main precursor for THMs formation.