Natural organic matter (NOM) is a complex mixture of various organic molecules mainly originating from aquatic organisms, soil and terrestrial vegetation which enters surface and ground waters. Through the different reaction reactivity of the NOM components with disinfectants, isolation and fractionation of NOM into more homogenous components is the better way of determining the DBP formation potential of the waters. Within the aim and scope of this paper, raw water samples were taken from Buyuk-cekmece (BC) and Omerli (OM) surface water sources which supply drinking water to Istanbul, in eight different months between February 2010 and February 2011. At first water quality parameters were analysed. Then, the physical structure of the waters' dissolved organic matter (DOM) content, seasonal variations of this structure, reactivity of DOM by means of DBP production and the effects of the seasonal variations on this reactivity were investigated. In terms of specific reactivity of the OM and BC physical fractions, <1kDa fraction was determined as the most reactive fraction for STHMFP and SHAAFP per mg DOM. >5kDa fraction was also determined as the most reactive part of the DOM in terms of SAOXFP for both waters. Both of the waters SUVA values were increased due to the increase of DOM molecular weight but there was no similar trend in the formation of by-products. The main reason for this situation is the lower SUVA and DOM values of the waters investigated. In this context, it can be said that OM and BC waters have special characteristics.