Ultrahigh-resolution negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize Athabasca oil sands processed water (OSPW) treated by (A) coagulation flocculent with lime and bentonite, (B) coagulation flocculent with lime and bentonite followed by activated carbon, and (C) combined ozonation and ultrasonication. Treatment A was ineffective in reducing the level of total naphthenic acid fraction components [NAFCs, defined as the acid-extractable fraction of OSPWs or crude oils (CnH2n + zOwSxNy), where the values n, w, x, and y indicate the number of carbon, oxygen, sulfur, and nitrogen atoms, respectively, and z represents the hydrogen atom deficiency because of the presence of double bonds and ring formation]. Likewise, for treatment A, little or no change was observed for the double bond equivalent (DBE) distributions of the compound classes. Treatments B and C resulted in the reduction of total NAFCs by 26 +/- 1.4 and 89 +/- 1.1%, respectively. For the latter treatments, there was evidence for selective removal of the S and OxSy heteroatomic species at the molecular level, along with a reduction in the DBE values for all species.