The photochemistry of aldehydes in the gas phase has been the topic of extensive studies over the years. However, for all but the smallest aldehydes the dynamics of the processes is largely unknown, and key issues of the mechanisms are open. In this article, the photochemistry of pentanal is studied by dynamics simulation using a semiempirical MRCI code for the singlet and triplet potential energy surfaces involved. The simulations explore the processes on the triplet state following intersystem crossing from the initially excited singlet. Test simulations show that the photochemistry takes place on the adiabatic triplet surface only and that no nonadiabatic transitions occur to the other triplets. The main findings include the following: (1) Norrish type I and type II reactions and H detachment have been observed. (2) The time scales of Norrish type I and Norrish type II reactions are determined: Norrish type I reaction tends to occur in the time scale below 10 ps, whereas the Norrish type II reaction is mostly pronounced after 20 ps. The factors affecting the time scales are analyzed. (3) The relative yield for Norrish type I and type II reactions is 34% and 66%, which is close to the experimental observed ones. Bond orders and Mulliken partial charges are computed along the trajectories and provide mechanistic insights. The results throw light on the time scales and mechanisms and competition between different channels in aldehyde photochemistry. It is suggested that direct dynamics simulations using semiempirical potentials can be a very useful tool for exploring the photochemistry of large aldehydes, ketones, and related species.