Mass spectrometry is a common tool for studying the fate of complex organic compound mixtures in oil sands processed water (OSPW), but a comparison of low‐, high‐ (∼10 000), and ultrahigh‐resolution (∼400 000) instrumentation for this purpose has not previously been made. High‐resolution quadrupole time‐of‐flight mass spectrometry (QTOF MS) and ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS), with negative‐ion electrospray ionization, provided evidence for the selective dissipation of components in OSPW. Dissipation of oil sands naphthenic acids (NAs with general formula CnH2n+zO2 where n is the number of carbon atoms, and Z is zero or a negative even number describing the number of rings) was masked (by components such as fatty acids, O3, O5, O6, O7, SO2, SO3, SO4, SO5, SO6, and NO4 species) at low resolution (1000) when using a triple quadrupole mass spectrometer. Changes observed in the relative composition of components in OSPW appear to be due primarily to the presence of plants, specifically cattails (Typha latifolia) and their associated microorganisms. The observed dissipation included a range of heteratomic species containing O2, O3, O4, and O5, present in Athabasca oil sands acid extracts. For the heteratomic O2 species, namely naphthenic acids, an interesting structural relationship suggests that low and high carbon number NAs are dissipated by the plants preferentially, with a minimum around C14/C15. Other heteratomic species containing O6, O7, SO2, SO3, SO4, SO5, SO6, and NO4 appear to be relatively recalcitrant to the cattails and were not dissipated to the same extent in planted systems. Copyright © 2009 John Wiley & Sons, Ltd.