The theoretical ab initio studies of the singlet states of salicylideneaniline (SA) are presented. The enol, cis-keto and trans-keto tautomers were treated by the HF/6-31G* (geometries and force fields of the ground states), and the CIS (excited states), methods. For the dynamic calculations of the rates of proton transfer (PT) in S-1 states, the instanton approach was applied. It was found that the SA molecule in S-0 and S-1 states of both tautomers needs nonplanarity to stabilize. In the ground state the corresponding angle was calculated as 44 degrees vs the experimental value, 49 degrees. Upon twist of the excited system, the conical intersection of (pi,pi*) and (n,pi*) potential surfaces takes place. In enol form the absolute minimum on the S-1 potential energy surface belongs to a strongly twisted (n,pi*) state. In keto-form this minimum corresponds to a planar (pi,pi*) state, while the twisted (n,pi*) has the energy approximate to 1055 cm(-1) higher. The angles of distortion are equal 93 degrees and 80 degrees, for the enol and keto form, respectively. Both (n,pi*) excited tautomers are practically unable to undergo the PT reaction. This, according to the calculations, goes via the planar unstable (pi,pi*) state. The calculated structures and force-fields lead to the (calculated) proton transfer rate of the order observed experimentally. The calculation of the transfer rate includes multimode nature of the transfer process. The PT cycle of the SA molecule involving the creation of photochromic transient has been proposed. (C) 2000 American Institute of Physics. [S0021-9606(00)30514-1].