The autoxidation of NO center dot was studied in glasslike matrices of 2-methylbutane at 110 K and in a 8:3 v/v mixture of 2,2-diinethylbutane and n-pentane (rigisolve) at 80-90 K, by letting gaseous NO center dot diffuse into these solvents that were saturated with O-2. In 2-methyllbutane, we observed a red compound. However, in rigisolve at 85-90 K, a bright yellow color appears that turns red when the sample is warmed by 10-20 K. The new yellow compound is a precursor of the red one and also diamagnetic. The UV-vis spectrum of the yellow compound contains a band which resembles that present in ONOO-. Because the red and yellow intermediates are not paramagnetic, we postulate that O=N-O-O-center dot is in close contact with NO center dot, or with another O=N-O-O-center dot. Diffusion of gaseous O-2 into rigisolve saturated with NO center dot does not produce a color; however, a weak EPR signal (g = 2.010) is observed. This signal most likely indicates the presence of ONOO center dot. These findings complement our earlier observation of a red color at low temperatures and the presence of ONOO center dot in the gas phase (Galliker, B.; Kissner, K; Nauser, T.; Koppenol, W. H. Chem. Eur. J. 2009; IS, 6161-6168), and they indicate that the termolecular autoxidation of nitrogen monoxide proceeds via the intermediate ONOO center dot and not via N2O2.