Coagulation experiments at pH values ranging from 3 to 7 were conducted on raw water samples from four Australian reservoirs-Hope Valley, Myponga, Moorabool and Mt Zero-to assess the removal of natural organic matter (NOM) with alum. The aim was to characterise the NOM in these water sources that is highly recalcitrant to removal by alum coagulation. The selection of these water sources covered a range in raw water quality varying in inorganic and organic composition and character. NOM in both raw and treated waters was characterised by several techniques including specific UV absorbance (SUVA), high performance size exclusion chromatography (HPSEC) and pyrolysis-gas chromatography mass spectrometry (Py-GC-MS). The results can provide better understanding of the removal limitations of each treatment step and the knowledge will allow design engineers to select a suitable combined treatment process for optimum NOM removal. Despite the fact that the organic character of the four source waters were different, results showed that after optimised alum coagulation all four waters had a similar character. The molecular weight distribution analysis (HPSEC) indicated alum coagulation preferentially removed the higher molecular weight UV absorbing compounds whilst those remaining in the treated waters had the properties of lower apparent molecular weights (about 500-700 Daltons) and less UV absorbance. Py-GC-MS analyses of NOM in these waters before and after treatment indicated that polysaccharides and their derivatives are recalcitrant to removal with alum coagulation. Generally, the findings indicate that the character of the NOM is an important factor in determining its treatability.