For cobyrinic acid heptamethylester derivatives, field desorption mass spectrometry can be used as a fast and sensitive analytical method for synthesis control in corrin chemistry. Since molecular ions are formed with high relative abundance, their identification in mixtures using accurate mass measurements and direct isotopic determination Is feasible. Detailed structural information can be obtained by thermally-induced fragmentation of the organometallic compounds on the emitter surface. Consecutive loss of the axial ligands and the substituent in position 10 of the corrin ring is observed with increasing sample temperature. Furthermore, at higher emitter heating currents, the product of the elimination of the acetic acid side-chain from C-7 is found analogously as in preparative thermolysis. The field desorption study presented here allows not only the detection of thermally generated end-products but also of reactive intermediates and thus gives a specific insight into the mechanisms of thermolysis. A preliminary comparison of the field desorption and fast atom bombardment mass spectra of some corrins is presented.