An improved procedure was used to isolate and fractionate natural organic matter (NOM) in water for subsequent ozonation and disinfection by-product (DBP) and color removal quantisation. Isolated NOM fractions from two different sources, accounting for approximately 50 to 60% of the dissolved organic material and 60 to 75% of the color, were characterized and then ozonated under conditions approximating those encontered during drinking water treatment. The natural waters also were ozonated. Organic DBPs of either health concern or which may contribute to biological instability of finished water were investigated, including aldehydes, oxoacids and low molecular weight carboxylic acids. pH and ozone dosage were the parameters having the greatest effect on DBP formation. On the basis of UV absorbance measurements, the fulvic acid fractions studied taken together accurately represented the natural water and may be the primary sources of precursor material for aldehydes and oxoacid DBPs. However, as yet unidentified NOM fractions contribute significantly to carboxylic acid formation upon ozonation.