The electron ionization tandem mass spectrometric analyses of six skeletal types of cycloalkanoporphyrins (CAP) together with their Ni(II) and VO(II) complexes are presented. Analysis of the daughter ion spectra of the molecular ions (M(.+)) indicates that it is possible to distinguish isomers with different sizes of isocyclic rings. For instance, deoxophylloerythroetio, DPEP, porphyrins with a five-membered isocyclic ring show a characteristic, intense (M - 44)(+) daughter ion, whereas CAP-6 porphyrins (CAPs bearing a six-membered isocyclic ring) display an intense (M - 45)(+) daughter ion. As a result of these findings, it is pow possible to identify the skeletal type of CAP porphyrins in intact or partially separated geoporphyrin mixtures. Data from previous studies an the porphyrins of New Albany shale (Mississippian-Devonian, Indiana) are used to demonstrate the potential applications of the current study.