Potassium peroxonitrite (ONOOK) and [Fe(EDTA)]2- were used to analyze the influence of chemically entirely different hydroxyl radical sources on tRNA cleavage profiles. [Fe(EDTA)]2- gives rise to hydroxyl radicals via a Fenton-like reaction during the oxidation of chelated Fe2+, while ONOOK generates hydroxyl radicals via its conjugate acid (ONOOH) when adding a stable alkaline solution of ONOOK in samples buffered at neutral pH. [Fe(EDTA)]2- is known to induce oxidative strand scission at sugar moieties thought to be solvent accessible, while those residues located in the 'inside' of structured RNAs are protected. Although ONOOH is neutral and significantly smaller than the metal complex, both reagents generate the same protection pattern on tRNAs, suggesting that access of the commonly formed hydroxyl radical, rather than access of its source, is the determining factor when probing the higher order structure of RNA. Strong difference in reactivity is only seen at the modified 2-thiouridine S34 of tRNA(Lys3) which shows hyperreactivity towards ONOOK treatment. This particular reaction may require interaction between the peroxonitrite anion and the thiocarbonyl group of the base, since hyperreactivity is not observed when probing the dethiolated tRNA(Lys3).