The EPR spin-trapping technique has been applied extensively to the detection of organic peroxyl radicals in biological systems. The most widely used spin trap is 5,5-dimethyl-1-pyrroline N-oxide (DMPO), of which adducts displaying EPR signals with a(N) ∼1.43 mT, a(β-H) ∼1.17 mT and a(γ-H) ∼0.12 mT have been routinely assigned to trapped peroxyl radicals. Recently, however, it has been shown that such signals are from alkoxyl radical adducts, generated during the decomposition of peroxyl radical adducts. In the present investigation, we have used the Ce^IV–tert-butyl hydroperoxide redox couple as an efficient means of generating peroxyl radicals (^tBuOO˙) in a fast-flow, dielectric mixing-resonator. This allowed the direct, EPR observation of ^tBuOO˙ radicals, as well as a short-lived radical adduct upon the inclusion of DMPO. Although the hyperfine coupling constants for this adduct were essentially indistinguishable from those of the more stable methoxyl radical adduct (DMPO–˙OMe), it is reasoned on kinetic and chemical grounds why this species is believed to be the tert-butylperoxyl radical adduct (DMPO–˙OO^tBu). The rate constant for ^tBuOO˙ spin trapping was estimated to be ca. 30 M⁻¹ s⁻¹, which is considerably lower than the value of > 10³ M⁻¹ s⁻¹ proposed recently by Honeywill and Mile (J. Chem. Soc., Perkin Trans. 2, 2002, 569), who concluded that alkylperoxyl radicals form only diamagnetic adducts, via their multiple addition to DMPO. Complementary spin-trapping experiments in a static system resulted in detection of the methoxyl and tert-butoxyl radical adducts of DMPO (generated via DMPO–˙OO^tBu decomposition), as well as the three-electron oxidation product 5,5-dimethyl-1-pyrrolidone-2-oxyl. These findings demonstrate that the chemistry underlying the generation of the radical adducts detected by spin trapping in peroxyl-radical generating systems must be interpreted with extreme caution. Furthermore, through the direct observation of the DMPO–˙OO^tBu adduct under continuous-flow conditions, this work gives support to earlier suggestions that DMPO peroxyl radical adducts are formed, but are too unstable to be detected under the conditions employed in typical spin-trapping studies.