The desorption ionization mass spectra are reported for two classes of synthetic porphyrins--'tailed' derivatives of deuteroporphyrin IX and meso-substituted derivatized tetraphenylporphines. The mass spectra were obtained by three desorption ionization techniques--fast atom bombardment (FAB), desorption chemical ionization (DCI) and 'in-beam' or desorption electron ionization (DEI). The emphasis of this study is to determine the usefulness of the three desorption ionization methods for the molecular weight and side-chain structure determinations of synthetic porphyrins. The molecular parent ions and the major fragmentation pathways are discussed in detail. The DEI method yielded the fewest mass spectra showing molecular parent ions from the compounds studied, whereas FAB gave the most. The major fragmentation pathways were compound dependent and similar for all three ionization methods; generally, those cleavages alpha and beta to carbonyl groups predominated. The FAB spectra showed strong molecular adduct ions in several spectra that could be attributed to an in situ coordination of iron-containing porphyrins with the thio-containing liquid matrix molecules.