On irradiation of p-(N-methyl-N-nitrosoamino)phenyldiazomethane (1) in an Ar matrix at 10 K, the liberation of nitric oxide (NO) to afford the quinoidal radical 2 was directly observed by means of IR spectroscopy. The photoisomerization of (E)-1 to its Z-isomer preceded the NO liberation. Warming the matrix containing NO and 2 caused their disappearance to give an EZ-mixture of the quinoidal imine 3 by the recombination of NO with a radical center of 2. The photoisomerization of (Z)-3 to (E)-3 was also observed.