Natural Organic Matter (NOM) was isolated and extracted from a moderately-colored surface water, then fractionated using hydrophobic and ion exchange resins into eight groups. Each was subjected to batch ozonation, and the rate of loss of ozone was measured. Then, selected fractions were analyzed for changes in functional group content, molecular size and hydrophobicity; some were further fractionated by size prior to functional group analysis. Fulvic acids and humic acids were the most reactive. These groups also showed the greatest change in carboxyl group content and hydrophobicity. The results are discussed with respect to impacts on coagulation. Although hydrophilic neutrals were of low to moderate reactivity, they showed the greatest promise for improved coagulation following preozonation.