AbstractOn light absorption in the long wavelength1La‐band 9‐alkyl‐substituted anthracenes in fluid solutions isomerise with low quantum yield (about 0.016 at 300 K) to the 9,10‐Dewarisomer, provided the substituent is sufficiently spacefilling (t‐butyl or t‐pentyl). On increasing the viscosity and/or reducing the temperature the quantum yield of this valence isomerisation decreases while the quantum yield of fluorescence and the vibrational structure of the fluorescence emission increases. From spectroscopic, caloric and kinetic data an energy diagram of the ground and first excited states is constructed for this photo‐ and thermoreversible reaction in which the folding along the 9,10‐axis of the anthracene sceleton serves as the main reaction coordinate. Steric interaction of the bulky substituent effects a folding of the anthracene sceleton along the 9,10‐axis already in the ground state, which was confirmed by an X‐ray analysis. Thereby a transition from the excited state to a hypersurface is facilitated on which a transformation in low yield to the Dewarisomer and mainly to the start