We have measured the reaction rate constants of the nitrone spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) with a number of small alkyl and sigma parent radicals in dilute aqueous solution using in situ radiolysis time-resolved electron spin resonance spectroscopy. Unsubstituted alkyl parent radicals (methyl, ethyl, propyl, and 1-methylethyl (2-propyl)) had rate constants ranging from 5.6 x 10(6) to 1.6 x 10(7) M-1 s(-1). Electron-releasing alpha-hydroxyalkyl radicals (hydroxymethyl, 1-hydroxyethyl, 1-hydroxypropyl, and 1-hydroxy-1-methylethyl (2-hydroxy-2-propyl)) reacted more rapidly than the unsubstituted radicals with rate constants of (2.2-6.8) x 10(7) M-1 s(-1), while the electron-withdrawing carboxymethyl radical was slower (4.4 x 10(6) M-1 s(-1)). The bulky 2-hydroxy-2-methylpropyl radical reacted with DMPO, but with a rate constant smaller than 10(6) M-1 s(-1). a radicals such as sulfite anion and carboxyl anion were trapped quickly, with rate constants of 1.2 x 10(7) and 6.6 x 10(7) M-1 s(-1), respectively. These results show that the zwitterionic structure of DMPO results in sensitivity to polar effects in the parent radical-spin trap encounter complex, while steric effects are also influential in the reaction of DMPO with bulky alkyl radicals. The rate constants for the reaction of DMPO with the radicals studied herein are, in general, an order of magnitude slower than the same radicals reacting with the nitroso spin trap 2-methyl-2-nitrosopropane.