Formation of aldehydes and trihalomethanes during intermediate ozonation of water pretreated with ozone and polyaluminium-chloride was monitored in a pilot-scale study to optimize the drinking water treatment technology. The raw water was the groundwater, rich in natural organic matter, originated from the territory of the Northern Banat region in Vojvodina Province, Serbia and Montenegro. Monitoring of the parameters that have to meet prescribed criteria (permanganate value, TOC and THMFP removal) could not give an indication on how to split the required ozone dose between preozonation and intermediate ozonation. As the effects obtained regarding THMFP and TOC removal were similar, monitoring of aldehydes was performed, and this clearly showed that preozonation should be carried out with a dose > or = 0.4 mg O3/mg TOC to obtain the water with the smallest contents of aldehydes and the lowest specific content of glyoxal in the GAC filter influent.