Dispersed fluorescence spectra (S-0<--S-1) of two conformational isomers of n-propylbenzene have been measured in a supersonic free jet expansion. The results show that the vibronic features in emission from the S-1 zero-point levels in two conformers are significantly different, and most notably, the transitions due to ring-chain torsional mode are active only in the spectra of the gauche conformer. Relative stability of the conformers in the ground state has been reinvestigated by the ab initio quantum chemistry method at the MP2/6-311++G(d,p) and MP2/ccpVTZ levels of theory. In contrast to earlier reports, the present theoretical studies predict that the gauche conformer is similar to2.5 kJ/mol more stable [MP2/6-311++G(d,p)] than the trans. The effects of propyl substitution on phenyl ring vibrational modes have been analyzed by comparing the calculated (ab initio, DFT/B3LYP/6-31G**) displacements of ring atoms for different normal modes with those of the vibrational modes of unsubstituted benzene. The implications of spectral dissimilarity of the conformers on their excited state lifetimes [Borst , J. Chem. Phys. 116, 7057 (2002)] have been discussed. (C) 2003 American Institute of Physics.