Positron annihilation lifetime spectroscopy (PALS) is used to study the reactions of the ortho-positronium (o-Ps) atom with the following nitroso spin traps: nitrosobenzene (NOB), N,N-dimethyl-4-nitroso-aniline (DMNA), and tert-nitroso-butane (t-NOBu). The reaction rate constants (k) are close to the range that corresponds to that of diffusion-controlled reactions and exhibit a similar trend to those reported in the literature for the analogous nitro compounds. The differences between the aromatic and aliphatic nitroso and nitro compounds and the observed substituent effect are in good correlation with the calculated LUMO energies of the molecules. The nucleophilic o-Ps attacks the molecules at their electron deficient aromatic ring. Solvent effects on the reaction rates in methanol-water solvent mixtures exhibit a complex and solute-dependent nature. The reactions are strongly affected by solvent viscosity; however, the expected 1/η versus k variation is valid only for limited ranges of solvent compositions. The dielectric permittivity effect on the reaction rate constants does not follow the expected trend; k values in water are significantly greater than in the less polar methanol. The average activation enthalpies (∆H q and entropies (∆S q ) evaluated for the reaction of o-Ps with DMNA exhibit maxima at a solvent composition which corresponds to the most rigid solvent structure, where the activation energies of the transport processes such as viscosity and self-diffusion have extremes too. The validity of the isokinetic relation between ∆H q and ∆S q shows that the mechanism of the reaction of o-Ps does not depend on the solvent composition. ∆H q exhibits an "Onsager reaction field" type ( -1)/( + 2) dependence on solvent permittivity. The results are compared to literature data for the reactions of o-Ps with stable nitroxide free radicals and reaction of solvated electron with nitrobenzene (NO 2 B) in methanol-water mixtures. The observed similarities and differences are discussed.

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