Laboratory tests presented in the article aimed to determine the influence of water hardness on the effectiveness of humic substances removal in the coagulation process. The tests were conducted on three model solutions; very soft water, soft water and semi-hard water. Concentration of humic substances in each solution was equal to 20 mg/dm3. Calcium hydroxide, ferric(III)chloride and aluminium sulphate(VI) were applied as coagulants. Based on the conducted tests, it was found that the increased content of calcium, and particularly of magnesium in water improves the effectiveness of humic substances removal. The process effectiveness expressed by changes in colour and permanganate index was the highest for semi-hard water. A decrease in colour equal to 88.8% and in permanganate index - 72.1% was observed in this case for the largest dose of calcium. Even better results were obtained by applying the remaining coagulants; a decrease in colour amounted to 99%, and in permanganate index to 95%.