The aim of the research was to evaluate the effectiveness of the coagulation process using pre-hydrolysed salts, polyaluminium chlorides, in reduction of water pollution with organic substances, including precursors to formation of trihalomethanes (THMs) during water chlorination. Surface water collected from the Stradomka river and from the Adriatyk reservoir located in Częstochowa was used for the research. In addition to typical indices used to assess the content of organic compounds (total and dissolved organic carbon TOC and DOC, oxygen consumption, ultraviolet absorbance UV254), the study also evaluated the potential of THM formation. In the laboratory conditions of the process using pre-hydrolysed coagulants, reductions in oxygen consumption and TOC contents depending on the month and place when water was collected were 17-42% and 14-38%, respectively, whereas reduction in UV254 absorbance ranged from 39 to 69%. The study confirmed the usefulness of the coagulation process in the removal of organic matter, which represents a precursor for formation of trihalomethanes. The potential for THM formation in treated water was by 28-55% lower than in untreated surface water after chlorination. The concentration of trichloromethane was 78-93% of the total THM content in the chlorinated treated waters. Dissolved organic matter played a major role in CHCl3 formation (76-88%).