Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S2 - Si deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, Si and S2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MRCISD methods. The photodynamical process starts with a fast (< 100 fs) planar relaxation from the S2 ππ* state into the π0π* minimum of the S2 state. The calculations demonstrate that two π-excited states (denoted ππ* and π0π* are actually involved in this stage. The time in reaching the S2/S1 intersections, through which thymine can deactivate to Si, is delayed by both the change in character between the states as well as the flatness of the S2 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 npi;* minimum of the Si state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the π0π* S2 trapping but also the trapping in the nπ* SI minimum contribute to the elongation of the excitedstate lifetime of thymine.