Vibrationally resolved electronic spectra of diphenylamine, three deuterated isotopomers, and both para-methyl and para-fluoro substituted derivatives have been recorded in a supersonic jet expansion using resonantly enhanced two-photon ionization. Analysis of these spectra, supported by ab initio calculations, has been used to determine the gas phase structure of diphenylamine. In both the ground and first excited singlet states, an effective C-2 symmetry structure is found in which the nitrogen atom is in a planar configuration and the phenyl rings adopt equal torsional angles. Calculations suggest that large-amplitude motion along the nitrogen inversion coordinate is possible in the ground electronic state. Isotopic substitutions have been used to assign the two low-frequency Franck-Condon active modes to different admixtures of symmetric phenyl torsion and bending about the central nitrogen. Electronic excitation to the S-1 state results in a decrease in the phenyl torsional angles of 7.4 degrees and an increase in the C-N-C bond angle of 4.0 degrees. While spectra of both the para mono-and dimethyl derivatives as well as the para-diflouro derivative indicate that little change occurs in either the physical or electronic structure of the basic chromophore, the spectrum of the monosubstituted para-fluoro derivative is indicative of a substantial perturbation to both. (C) 1997 American Institute of Physics. [S0021-9606(97)00447-9].