In this paper we report on the time‐resolved fluorescence decay lifetimes from photoselected vibrational states in theS1manifold oftrans‐stilbene, 4‐Cltrans‐stilbene and 4‐methyltrans‐stilbene cooled in supersonic jets and excited by a mode‐locked dye laser. Decay lifetimes as short as 100±30 ps were recorded, using a fast single photon counting system. The energy dependence of the nonradiative decay rates,knr, oftrans‐stilbene is invariant with respect to the para‐substitution of Cl, whereupon the intersystem crossing to the triplet in 4‐Cltrans‐stilbene dose not provide the main route for photoisomerization and constitutes a distinct, energy dependent, decay channel. The invariance ofknrwith respect to para‐methyl substitution indicates that intrastate vibrational energy redistribution (IVR) does not involve all accessible molecular modes. The experimental data are consistent with a nonadiabatictrans–cisisomerization in the isolated molecule, which involvesS1–S2internal conversion in conjunction with IVR.