Solution vibrational spectra of trans-stilbene and its 12 isotopomers have been recorded. Several key vibrations of the ground electronic state (S o ) which are sensitive to trans-cis isomerization are analyzed in detail in terms of the potential energy distribution (PED) and/or harmonic mode scrambling matrices (HMS). Generally, the vibrational mixing is weak for the majority of ring vibrations. Those for which it is strong are determined by the closeness of their and the bridging groups' intrinsic frequencies. The observed changes in the distribution of vibrational intensities in the 1600−1200-cm −1 region are explained in terms of vibrational mixing