The vibrational and electronic spectra of the stable pentalene derivative 1,3,5-tri-tert-butylpentalene (TTBP) are reported, and their properties are discussed on the basis of QCFF-PI MO semiempirical calculations, which allow geometry optimization and normal mode analysis for the ground and lowest excited states. The infrared and Raman spectra have intensities mostly arising from modes of pentalene origin. The observed frequencies are compared with those obtained from vibrational calculations on the parent molecule and on several tert-butyl derivatives. The pi pi* electronic transitions of TTBP in the visible and UV regions are satisfactorily predicted by our calculations, including the interaction between singly excited configurations. Optimization procedures show that bond alternation, a distinctive feature of antiaromatics, is reduced in the excited states and that ground and excited state potential surfaces have minima displaced one with respect to the other. Low-temperature absorption spectra in the S-2 and S-3 regions are interpreted in terms of Franck-Condon vibronic transitions Whose strength depends directly on the structural change upon excitation. The theoretical results on S-0 --> S-2 and S-0 --> S-3 band profiles, with consideration of the normal mode rotation in the excited state, that is, the Duschinsky effect, are in fair agreement with experiment.