By exciting cyclohexene in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization with mass-selective detection of the ion yields, we found four time constants tau (i) (20, 47, 43, 350 fs). Whereas deuteration lengthens tau (2) by a factor of 1.4, the other constants do not change. tau (1)-tau (3) represent traveling times through observation windows on excited surfaces, whereas tau (4) reflects a process in the hot ground state. We assign tau (1) (20 fs) to departure from the Franck-Condon regions of the Rydberg and pi pi* states, which are both populated at 200 nm, and tau (2) (47 fs) to traveling along the pi pi* surface and suggest that a [1,3]-sigmatropic H shift begins in this state. This rationalizes the deuterium effect on 2. To explain why this window is followed by a process not subject to a D effect, we postulate that the pi pi* surface is crossed late (i.e., at low energy) by the zwitterionic state Z and that formation of a carbene (the known photochemical product, cyclopentylcarbene) begins there. The corresponding 1,2-shift of a CC bond is then (within tau (4) = 350 fs) largely reversed on the ground-state surface, while a smaller part of the carbene forms products such as methylenecyclopentane within the same time. Carbene formation is probably accompanied by some cis-trans isomerization. The wavelength dependence of carbene formation is attributed to a memory for the initially excited state, based on momentum conservation. The processes are most likely typical of simple olefins. The fragmentation pattern showed that butadiene is not formed until at least 500 ps. The retro-Diels-Alder reaction, known to take place in the ground state, thus only occurs later.