Thermal decomposition of potassium persulphate (K2S2O8) was studied in detail by the EPR spin trapping technique in dimethylsulphoxide (DMSO), employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide (EMPO) and 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DIPPMPO) as spin traps. DMPO and/or its derivatives exclusively trapped the primary formed SO4*- radical anions producing stable spin adducts with half-lives exceeding 2 h at room temperature. High-resolution EPR spectra of these adducts showed unusually rich hyperfine structure due to the interaction of the unpaired electron with all magnetically active nuclei of the spin trap moiety. In contrast to aprotic DMSO solvent, *DMPO-OH adducts dominated in mixed DMSO/water solutions with water content higher than 50%. The thermal decomposition of K2S2O8 in DMSO represents an effective source of free radicals for the radical scavenging capacity (RSC) determination assay, applicable to hydrophilic as well as hydrophobic antioxidants. Efficiency of the assay is demonstrated with a series of cereal samples.