Abstract The near u.v. absorption spectra of eight phenyl substituted chloro- and fluorocyclotriphosphonitriles have been measured at 23°C. The spectra are related to those of monosubstituted benzenes and of benzene itself. Vibrational structure is visible in the 2600–2800 Å region, especially for the compounds with low phenyl content. The intensities and positions of the absorption maxima in the higher wavelength region indicate some, although no very large, conjugation between the phenyl groups and the phosphonitrilic ring. There are some interesting differences in the u.v. spectra of the compounds studied, but attempts to explain these differences fail at present.