It is confirmed by measurements of fluorescence spectra and quantum yields that the fluorescence in tolan originates from the same state that causes the absorption band at lowest energy. The temp. dependence of the fluorescence quantum yield shows that this state is thermally deactivated with an activation energy of EA = 14.0 kJ/mol. Geometry optimizations of the states S0, S1, and T1 of tolane with the semiempirical AM1 method lead to planar structures with D2h symmetry. Potential energy curves along the triple-bond stretching coordinates have been calcd. for several low-lying excited states with a combination of the AM1 and the INDO/S methods. It is found that for large triple-bond lengths, the 1Au-state with sp* character becomes the lowest excited singlet state. It is proposed that thermal deactivation of S1(11Blu) leads to this state. Nonvertical excitation of 11Au could explain the weak lines found in supersonic jet expts. below the onset of the 11Ag -> 11Blu transition.