In this paper we perform ab initio calculations for the stable conformations and the transition states for the isomerization processes in 5-hydroxytropolone in both the ground (S-0) and first excited (S-1) singlet electronic states. The Hartree-Fock self-consistent field (SCF) level and a complete active space SCF (CASSCF) level for S-0 are considered, whereas the configuration interaction all single excitation method (CIS) and the CASSCF levels are used to deal with the S-1 state. Energies are reevaluated at all levels through perturbation theory up to second order: Moller-Plesset for the Hartree-Fock and CIS methods, and the CASPT2 method for CAS results. The ab initio results are then used to perform different monodimensional fits to the potential energy surfaces in order to analyze the wave functions for the nuclear motions in both electronic states. Our best results predict that for the S-0 state two stable conformers, syn and anti, can exist in thermal equilibrium. In accordance with experimental expectations the syn isomer is the most stable. As for the S-1 state, and again in accord with experimental spectroscopical data, the order of stability reverses, the anti being the most stable. A more interesting result is that analysis of the nuclear wave functions shows an important syn-anti mixing in the S-1 state that does not appear in S-0. This result explains the appearance of syn-anti and anti-syn crossover transitions observed in the electronic spectra of 5-hydroxytropolone so that syn-anti reaction may take place through photoisomerization. (C) 1997 American Institute of Physics.