Electron paramagnetic resonance (EPR) spectroscopy is the ideal method of choice when detecting and studying the wide variety of paramagnetic oxygen-centred radicals. For simple diatomic radicals, such as the superoxide (O 2 −) or peroxy (ROO •) species, the CW EPR profile (in particular the g-values) of these species can appear similar and indeed indistinguishable in some cases. Experiments using 17 O-enriched oxygen, revealing a rich 17 O hyperfine pattern, are therefore essential to distinguish between the two species. However, in many cases, particularly involving TiO 2 pho-tocatalysis, the peroxy-type (ROO •) radicals or other intermediate species such as the [O 2 − …organic]-type adducts can be transient in nature and once again can produce similar g-values. In general terms, these reactive oxygen species (ROS) are formed and detected at low-temperature conditions. Hence, the application of EPR spec-troscopy to studies of surface-stabilised oxygen-centred radicals must be performed under carefully selected conditions in order to confidently distinguish between the differing types of diatomic radicals, such as O 2 − , ROO • or [O 2 − …organic].