Theoretical and Raman spectroscopic studies are presented of the ground and lowest excited triplet states of (E)- and (Z)-2,5-dimethyl-1,3,5-hexatriene and their 3,4-dideuterio and 3-deuterio isotopomers. The T1 potential energy surface is calculated from SCF-LCAO-MO-CI theory. Energy minima and equilibrium geometries are determined in the T1 state. Frequencies and normal modes of vibrations are calculated for the minima of the T1 and S0 states. Energies of higher triplet levels are computed and oscillator strengths for the T1 --> T(n) transitions are determined. The displacements in equilibrium geometries between the T1 and the T(n) level corresponding to the strongest T1 --> T(n) transition are calculated and are used to estimate the intensities of the resonance Raman spectra of the T1 state under the assumption of a predominant Franck-Condon scattering mechanism. The influence of the ground-state conformation around C-C single bonds on T1 resonance Raman spectra is considered in detail for the two isomers. It is found that for the E isomers the tEt and for the Z isomers the tEc forms are the predominant ones in the T1 state. The Z forms are at considerable higher energy than the E forms in the T1 state due to nonbonded interaction. A good agreement is found between theoretically calculated and experimental spectra. The results are compared with previously published data for 1,3,5-hexatriene.