\ The mechanisms which are responsible for the radiationless deactivation of the n pi* and pi pi* excited singlet states of thymine have been investigated with multireference ab initio methods ( the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the (1)n pi* and (1)pi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S-1 and S-0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest (1)pi pi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the (1)pi pi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark (1)n pi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.