Ab initio nonadiabatic dynamics simulations arc reported for thymine with focus on the S-2 -> S-1 deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S-1 and S-2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (<100 fs) planar relaxation from the S-2 pi pi* state into the pi(0)pi* minimum of the S-2 state. The calculations demonstrate that two pi-excited states (denoted pi pi* and pi(0)pi*) are actually involved in this stage. The time in reaching the S-2/S-1 intersections, through which thymine can deactivate to S-1, is delayed by both the change in character between the states as well as the flatness of the S-2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the n pi* minimum of the S-1 state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the pi(0)pi* S-2 trapping but also the trapping in the n pi* S-1 minimum contribute to the elongation of the excited-state lifetime of thymine.