In the past decade, a number of UK and US water utilities have been experiencing operational difficulties connected with the increased dissolved organic carbon (DOC) levels during the autumn and winter periods. This has been observed as an increase in the production of disinfection-by-products (DBP), and a greater coagulant demand. Resin adsorption techniques were used to fractionate raw water and investigate the variation in surface charge and coagulant-humic interactions over a 36-month period. A change in the natural organic matter (NOM) composition throughout the year was observed, with the fulvic acid fraction (FAF) increasing from 36% in September to 61% in November. However, a reduction in treatment performance is not simply due to an increase in DOC concentrations (from 4.3 to 14.5 mg L-1), but also a change in the charge density of the NOM. It was found that hydrophilic NOM fractions possess negligible charge density (<0.06 meq g-1DOC), and it is the hydrophobic NOM fractions, FAF in particular, that exert the greater dominance on coagulation control. The hydrophilic NOM fractions are less amenable to removal through conventional coagulation with metal salts, and are therefore likely to indicate the DOC residual remaining after treatment. Understanding the seasonal changes in NOM composition and character and their reactivity with coagulants should lead to a better optimisation of the coagulation process and a more consistent water quality.