The laser-induced fluorescence excitation spectra of jet-cooled 4,4'-dimethyl-trans-stilbene have been recorded, and the phenyl and methyl internal rotation levels have been assigned for the S-0 and the S-1(pi,pi*) electronic states. Dispersed fluorescence spectra and vapor-phase Raman spectra were utilized to help assign the low-frequency vibrations in the electronic ground state. The methyl torsions have energy levels and potential functions very similar to those of m-xylene. The S-0 state has V-6 = 15+/-15 cm(-1) (negligible barrier), while S-1 has V-3 = 85 +/- 10 cm(-1) and V-6 = -30 +/- 20 cm(-1). The barrier to simultaneous internal rotation of the phenyl groups in So was found to be 3100 cm(-1). This is identical to that of trans-stilbene within experimental error. For the S, state, the barrier is 3140 cm(-1) as compared to 3000 cm(-1) for trans-stilbene. The barrier to rotation about the C=C bond in the S-0 state was estimated to be 41 +/- 5 kcal/mol based on one assigned frequency. In the S-1 state the trans-twist barrier was estimated to be approximately 2400 cm(-1).