Synthesis and spin trapping chemistry of the new C-13-labeled cyclic nitrone, 2-phenyl-5,5-dimethyl-1-pyrroline N-oxide (nitronyl-C-13), is described. A total of 15 carbon- and oxygen-centered radical adducts were prepared and their electron paramagnetic resonance (EPR) spectral parameters examined in organic as well as aqueous solutions. All the radical spin adducts derived from the novel nitrone displayed a well-resolved triplet of doublets in the EPR spectra. The alpha-C-13 hyperfine splitting is a good indicator for the chemical structure of the added radical and varies from around 3.2 to 6.2 G. Free radicals are spin trapped at rates that are quite comparable to those observed with C-phenyl-N-tert-butylnitrone (PBN). The main advantage, however, is the enhanced persistence of its radical spin adducts. For this reason the title nitrone holds promise as a biological spin trapping agent because spin adduct decay during metabolism or isolation often governs the success or failure of a given experiment.