It has been shown by different experiments using field ion mass spectrometers that molecules may be dissociated by very high electric fields (several 107 - 108 V/cm) immediately after field ionization. The large variety of field dissociation processes observed in field ion mass spectrometers is treated systematically. This is done by assuming a basic model underlying the effect of field dissociation. The rules derived from the model are confirmed experimentally by the field ion mass spectra of homologous series of organic substances. After derivation of the model it is shown that field dissociation of organic ions is dependent on factors such as: Charge distribution in the molecule rearranged by the electric field, interaction of the positively charged parts of the molecule with the external electric field, internal interaction of the field dissociating parts of the molecule. Each of these main factors in turn is dependent on further factors which will be discussed, the most important ones being the mass to charge ratio and the electronic structure of both the field ionized molecule and the subsequently formed fragments.