No ultraviolet spectra of 1,3-diphenyl-2,4-uretidinedicne (phenyl isocyanate dimer) and triphenyl-s-triazine-2,4,6-trione (phenyl isocyanate trimer) are recorded in the literature at present. Infrared spectra have been shown to serve as a basis for the selective analysis of these trimers and dimers. It is now apparent that ultraviolet spectra may also serve to distinguish the trimers from the dimers. The ratio of the molar absorptivities of the two bands of each compound (the E1 and E2 bands of benzene) provides the suitable and practical method for this selection.Consideration of the following has led to conclusions concerning the structure of dimers and trimers: (1) spectra of benzene, monosubstituted benzenes, and disubstituted benzenes; (2) wavelength position the dimers and trimers (E2 band of stituent effects on the spectra of The primary structural implication of the secondary band of benzene); and (3) subthe dimer and the trimer. uncovered is that the four membered heterocyclic ring of the dimer prefers to remain unconjugated, while conjugation of the six membered heterocyclic ring of the trimer seems to be extremely important.Large ortho substituents in the trimer provide a considerable steric hindrance and consequently the spectra is influenced almost exclusively by said substituent and the benzene ring. Trimers with only hydrogen in the ortho position are also found to be sterically hindered from participation with the six membered heterocyclic ring, but to a much smaller degree.