The electronic structures of the ground and the lowest excited states of catechol and its 1:1 complex with aluminium have been optimized by taking into account the solvent effects. The good agreement between the theoretical absorption and fluorescence emission wavelengths with the experimental ones has allowed the validation of the computed electronic structures. If the structure of the S-0 state of catechol is planar, it is twisted in the S-1 state in water, contrary to the results obtained in vacuum. In opposition, the changes observed for the 1:1 complex structures in the S-0 and S-1 states are negligible. (C) 2010 Elsevier B. V. All rights reserved.