Several vibronic bands that appear within 300 cm(-1) of the electronic origin in,the S-1<--S-0 fluorescence excitation spectrum of 1-naphthoic acid (1NA) and carboxyl-deuterated 1NA have been examined at full rotational resolution. The data show that all bands belong to the s-cis isomer of 1NA. They also show that all bands are torsional in-nature; i.e., that they involve displacements along either the S-0 or the S-1 carboxyl torsional coordinate, phi, or both. Unambiguous assignments of the bands follow from the observed inertial defects, from which the torsional potential energy surfaces of both electronic states are derived. In S-0 s-cis-1NA, the naphthalene and carboxyl groups are not coplanar. The s-cis minima are at phi= +/- 17 degrees; the barrier to planarity is 13.2 cm(-1). In contrast, S-1 s-cis-1NA is a completely planar molecule, with phi=0 degrees. The barrier for s-cis to s-trans isomerization is similar to 1450 cm(-1) in the S-0 state and similar to 2200 cm(-1) in the S-1 state. The validity of the two derived potential energy surfaces of s-cis-1NA has been tested by comparing the observed inertial defects with those computed using the torsional eigenfunctions. Excellent agreement is obtained, confirming the one-dimensional approach em ployed.