Intersystem crossing is the crucial first step determining the quantum efficiency of very many photochemical and photophysical processes. Spin-crossover compounds of first-row transition metal ions, in particular of Fe(II), provide model systems for studying it in detail. Because in these compounds there are no competing relaxation processes, intersystem crossing rate constants can be determined over a large temperature interval. The characteristic features are tunnelling at temperatures below ∼80 K and a thermally activated process above ∼ 100 K. This, as well as the twelve order of magnitude increase of the low-temperature tunnelling rate constant on going from a spin-crossover compound with a small zero-point energy difference to a low-spin compound with a substantially larger one, can be understood on the basis of a nonadiabatic multiphonon process in the strong vibronic coupling limit.