Diphenylamine (DPA) on illumination with natural sunlight or artificial UV-light, in presence of air, transforms into N-phenyl-p-benzoquinonimine (PBQ). This direct photooxidation exhibits saturation kinetics with respect to DPA and air. Increase of the area of sunshine linearly improves the formation of PBQ. The UV-photooxidation rate is a linear function of photon flux and the quantum efficiency is low with illumination at 365 nm but is significant at 254 nm. Singlet oxygen quencher, azide ion, does not suppress the photooxidation. Sacrificial electron donors, vinyl monomers and micelles, both anionic and cationic, do not interfere in the photoreaction. The phototransformation of DPA to PBQ depends on the solvent employed; occurs in ethanol, n-butanol, ethyl acetate, ethyl methyl ketone, 1,4-dioxane, acetonitrile, chloroform and carbon tetrachloride, no reaction in n-hexane as well as in benzene and acetic acid, chlorobenzene, and toluene led to different products. The phototransformation is discussed in terms of a mechanism and kinetic law. (c) 2006 Elsevier B.V. All rights reserved.