The S1–S0 fluorescence excitation spectrum of 4-methoxystyrene seeded into a supersonic free jet shows two 000 bands, corresponding to the cis and trans rotational isomers. Dispersed fluorescence, with excitation in each of these bands, show that some of the resulting ground-state vibration wavenumbers are appreciably different. This confirms the 000 band assignments to different isomers. One prominent vibration has a wavenumber of 373 or 400 cm–1, depending on the isomer involved. These two bands are also observed in the Raman spectrum of the liquid and the fact that the 373 cm–1 band disappears in the solid-state Raman spectrum at 269 K identifies the 000 band of the more stable isomer. Ab initio calculations at the 6-31G* level indicate that the trans isomer is the more stable but by only 77 cm–1. This compares with a possible value of 174 cm–1 derived from experimental data.