The efficient population of the low-lying triplet pi pi* state of psoralen is studied with the quantum chemical CASPT2 method. Minima, singlet-triplet crossings, conical intersections, and reaction paths on the low-lying singlet and triplet states hypersurfaces of the system have been computed together with electronic energy gaps and spin-orbit coupling terms. A mechanism is proposed, favorable in the gas phase, for efficient deactivation of the initially populated singlet excited pi pi* state, starting with an intersystem crossing with an n pi* triplet state and evolving via a conical intersection toward the final lowest-lying pi pi* triplet state, protagonist of the reactivity of psoralen. (c) 2006 Elsevier B.V. All rights reserved.