Dissolved organic matter (DOM) in oil refinery process water was fractionated by XAD-8 resin techniques into four subfractions: hydrophobic acid (HOA), hydrophobic base (HOB), hydrophobic neutral (HON), and hydrophilic substance (HIS) fractions. Negative and positive electrospray ion (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize the composition of DOM and its subfractions. Compounds with multi-oxygen atoms were found to be predominant in DOM by either negative or positive ESI analysis, which are similar in composition to most other treated water samples. The DOM in the HOA fraction had a similar molecular composition to that of raw process water by negative ESI analysis. The DOM in the HOB fraction had a low molecular weight (MW) when analyzed by positive ESI, and basic nitrogen compounds, such as N-1 class species, were found to be predominant. The DOM in the HON fraction was predominantly O-2 class species. The DOM in the HIS fraction had a relatively wide MW distribution. All of the compounds of DOM in the HIS fraction exhibited low double bond equivalents (DBE) and low carbon numbers. The results showed that the use of the XAD-8 resin fractionation technique is valuable for characterizing trace quantities of DOM components in process water because their spectral peaks would otherwise be obscured by other abundant peaks. The origin and determination of chlorine-containing compounds, which are abundant in the negative ESI mass spectra of HOB, were discussed.