Naphthenic acids (NAs) are concentrated in oil sand process water (OSPW) as a result of caustic oil sands extraction processes. There is considerable interest in methods for treatment of NAs in OSPW. Earlier work has shown that the combination of ultraviolet (UV) and microwave treatments in the laboratory was effective in reducing the concentration of classical NAs. Here we apply Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to further characterize NAs treated with (a) UV (254 nm) in the presence of TiO2 catalyst; and/or (b) microwave irradiation (2.45 GHz). FT-ICR MS was used to characterize the NA fraction before and after treatment. Acidic oxygen-containing classes were most abundant in all samples whereas other heteroatomic classes were least abundant or not present in some samples. For example, the SO2-containing species were absent in UV- or combined UV- and microwave-treated samples. The O2 class was dominant in all samples, indicative of NAs. However, samples treated with UV and microwave radiation have a lower relative abundance of other heteroatomic classes. We observed O2, S1O2, O3, S1O3, O4, O5, and O6 classes, whereas the species with relatively high On content, namely, the O3, O5, and O6 classes, were present only in UV- and microwave-treated samples. The relatively high On content is consistent with oxidation of the parent acids in treated samples. There may thus be potential implications for environmental forensics. For example, the monitoring of the ratio of SO2:O2 or tracking the relative abundances of O2, O3, O4, O5, and O6 classes may provide insights for distinguishing naturally derived oil sands components from those that are process-related in aquatic environments. Copyright © 2010 John Wiley & Sons, Ltd.