The lifetimes of the 3-methylindole-NH3 complex have been measured on different vibronic levels involving intermolecular modes and decrease from 530 to 65 ps, in a mode specific manner. Geometry optimizations of the ground and excited states have been performed with ab initio methods, and as in the case of phenol and indole, a repulsive pi sigma* state lies close to the initially excited pi pi* state. From these calculations, it seems that both in-plane and out-of-plane vibrations induce a faster nonradiative decay.