Reaction rate constants between OH radicals and the dimer and monomer of the spin trap 2-methyl-2-nitrosopropane (MNP) were determined by the pulse radiolysis method. Before pulse radiolysis experiments, the molar absorbance coefficients of the dimer and monomer were determined by a method combining NMR and optical absorbance spectrometries. First, NMR signal intensities of the dimer and monomer were related to their molar concentrations using deuterated tert-butyl alcohol, (CH3)(3)C-OD, as a standard. Since the monomer was eliminated by bubbling the solution with Ar gas and the remaining dimer was found to gradually decompose to the monomer, the changes of molar concentrations against time starting from the dimer alone until reaching dimer-monomer equilibrium were next measured by monitoring both NMR and optical absorbance spectra (at delta = 1.59 and lambda = 320 nm for the dimer and at delta = 1.28 and lambda = 662 nm for the monomer). The molar absorbance coefficients were finally determined to be epsilon(320) (nm) = 410 +/- 40 M(-1) cm(-1) for the dimer and epsilon(662) (nm) = 10 +/- 5 M(-1) Cm-1 for the monomer by comparing the curves measured by NMR with those measured by optical absorbance spectrometry. Pulse radiolysis experiments on the reaction rate constant between OH radicals and the dimer were carried out in aqueous solutions containing different amounts of the MNP dimer alone, and it was determined to be (3.0 +/- 0.3) x 10(9) M(-1) s(-1). Experiments on the reaction rate constant between OH radicals and the monomer were carried out in solutions containing both the dimer and monomer, and it was determined to be (1.3 +/- 0.9) x 10(10) M(-1) s(-1).