In the present work, the ultrafast dynamics of the photoinitiated electrocyclic ring-opening reactions of 1,3,5-cyclooctatriene and 7-dehydrocholesterol are investigated in a fs pump-probe experiment using ionization as the probe step. Concerning the 1,3,5-cyclooctatriene photoreaction, there are strong arguments for the participation of two different reaction channels (involving a 3-center and a 4-center substructure) which lead to three products. Studying the ring-opening reaction of 7-dehydrocholesterol, which is a substituted cyclic polyene, answers the crucial question to what extent an ultrafast reaction can be slowed down by the influence of a steric factor
