The kinetics of unimolecular decomposition of organic ions in the presence of electric fields up to 108 V/cm is discussed. The results are deduced from the mass spectra obtained by field ionization of organic molecules. The time interval between formation and decomposition of the ions can be derived from the shape of the mass lines. It is shown that field induced decomposition can occur at a minimum time of about 3 · 10–14 sec after field ionization. The optimum time resolution with these experiments, 10–12 sec, is given by the resolving power of the mass spectrometer. The results can be interpreted in terms of five different mechanisms : 1. Field induced dissociation. Excitation of the molecular ion to a state above the dissociation limit is possible at high enough fields. Spontaneous dissociation within one vibrational period will then be possible because the excitation energy is contained in the reaction coordinate. 2. Field induced dissociation by tunneling of radicals through the potential barrier. This may occur after excitation of the molecular ion to a state lying in an interval between the dissociation limit and about half a vibrational energy quantum below this, provided the field strength is high enough. The maximum observable life time of ions resulting from this process is about 6·10–12 sec. 3. Field induced dissociation, delayed by re-orientation of the molecular ion. Certain orientations of the molecular ion are favoured with respect to field dissociation. The maximum re-orientation time for a favourable position is of the order 3·10–12 sec. 4. Field induced statistical dissociation. This is due to fluctuation of energy within the ion, as in the case of common reaction kinetics, but with lowering of the dissociation limit by the field. The time intervall for these processes lies between about 10–13 and 10–11 sec. 5. Statistical decomposition in the space of low or zero field strength. This process is due to energy fluctuation within the molecular ion excited to a state above the dissociation limit, lowered only by a minute amount by the weak field. The range of life times is about 10–11 to 10–8 sec. Processes of the same type, occurring at zero field strength within about 10–8 and 10–6 sec, are called — as usually — “metastable” processes. The processes described here are derived mainly from the FI mass spectra of paraffins, alcohols, ethers and ketones. Dissociation is hindered, in some cases, by high electric fields.