Hardness cations are ubiquitous and abundant in source water, while the effect of hardness on the performance of coagulation for dissolved organic matter (DOM) removal in water treatment remains unclear due to the limitation of methods that can characterise the subtle interactions between DOM, coagulant and hardness cations. This work quantified the competition between coagulant Al3+ and hardness cations to bind onto DOM using absorbance spectroscopy acquired at different Al3+ concentrations in the absence and presence of Ca2+ or Mg2+. The results indicate that, in the presence of either Mg2+ or Ca2+, an increasing depression of the binding of Al3+-DOM could be observed in the differential spectra of DOM with the increasing of Mg2+ or Ca2+ at a level of 10, 100 and 1000 mu M, with the observation being more significant at higher pH from 6.5 to 8.5. The results of zeta potentials of DOM indicate that the competition of hardness cations results in the negative DOM being less efficiently neutralised by Al3+. This study demonstrates that the removal of DOM by coagulation would significantly deteriorate with the presence of hardness cations, which would compete with coagulant Al3+ to neutralise the unsaturated sites in DOM. (C) 2017 Elsevier Ltd. All rights reserved.