Complex reactions involve the transformation of molecular structures with many atoms. Ultrafast electron diffraction is developed here to directly observe such transient structures. The level of complexity is demonstrated for both the thermal and light-mediated reactions involving the electrocyclic Cope bond rearrangement and ring opening of 1,3,5-cyclooctatriene. For the former, we determine the ground-state structures, and for the later we observe the transient structures in their non-equilibrium state. The selective structural determination reported here using pulse sequences would not have been possible without the high sensitivity and spatio-temporal resolution of ultrafast electron diffraction. (C) 2003 Elsevier Science B.V. All rights reserved.