We report quantum chemical semiempirical calculations of the electronic structure of serotonin (5-hydroxytryptamine, 5-HT) and 5,7-dihydroxytryptamine (5,7-DHT). Those substances are important in the context of Alzheimer's disease. Moreover, the tautomerism of 5,7-DHT, which is not present in serotonin, is also examined. We have found that the phenol form of 5,7-DHT is more stable than is the 4-keto form in the gas phase. However, the large dipole moment of the keto form suggests that a large stabilization of this form can occur in a polar environment, which is confirmed by further supermolecule calculations. We have also calculated the absorption spectra of both 5,7-DHT and serotonin (5-HT) using the INDO/S method at the calculated AM1 geometries, with good agreement with experimental data. Our results give strong support to the hypothesis of the phenol–keto tautomerism of 5,7-DHT being responsible for the UV-visible spectroscopic features. © 1993 John Wiley & Sons, Inc.