Laser jet spectroscopy is applied to study the vibronic activity in (beta)-methylnaphthalene and phenanthrene. Results of these studies indicate that the technique is invaluable in the determination of the mechanisms and spectral manifestations of vibronic coupling in large polyatomic species;In the S(,1) state of (beta)-methylnaphthalene, a Duschinsky rotation of normal coordinates results in extensive mode mixing as evidenced by highly structured single vibronic level (SVL) fluorescence spectra and drastic absorption/fluorescence mirror symmetry breakdown (MSB). The rotation is shown to be well modeled by simple first order perturbation formulae which readily reproduce the observed spectra. Additionally, anharmonicities to fourth order are apparent from the spectra of the molecule;In phenanthrene, absorption/fluorescence MSB is shown to be primarily the result of Condon/Herzberg-Teller transition moment interferences and model calculations again indicate that simple first order formulae can be used to reproduce the observed spectra. While mode mixing does not appear to be important for totally symmetric modes of the molecule, vibronically induced transitions of b(,2) symmetry clearly indicate mixing from a Duschinsky rotation;in these normal modes. Mixing as a result of Fermi resonances is also observed for some levels; *DOE Report IS-T-1205. This work was performed under Contract W-7405-eng-82 with the U.S. Department of Energy.