A modification of the basic Urey—Bradley force field, which has been shown to reproduce the planar vibrational spectra of benzene and substituted benzenes, is extended to naphthalene. An alternate modification based on MO theory is compared with the simpler Kekulé model, and both models are shown to lead to the same conclusions regarding the vibrational spectra. A zero-order calculation using thirteen force constants transferred from other molecules agrees remarkably well with the recent Scully—Whiffen and Freeman—Ross assignments. In the B2u species, where these authors have serious differences, the modified UBFF favors the Scully—Whiffen assignment. A 16-parameter UBFF is refined to the H8 and D8 isotopes, and the vibrational frequencies of all deutero isotopes having Vh, C2v, or C2h symmetry are calculated with this potential function.