The extensive macrocycle-but minimal substituent-fragmentation occurring in the chemical ionization mass spectra of porphyrins when ammonia, rather than hydrogen, is the reagent gas leads to more detailed structure determinations. Conventional electron-impact mass spectrometry (EIMS) has proved to be of' relatively little value for the structural elucidation of porphyrins for two reasons. 2 First, the macrocycle does not cleave to any appreciable extent; thereby, necessarily, precluding gaining any information about the substituents attached to particular pyrrole rings. Second, while some diagnostic substituent cleavages do occur (most notably, loss of .CH2C02R from propionic ester groups) other important groups (e.g., vinyl and formyl) are silent under EIMS conditions. Although the advent of GC/MS techniques encouraged the return to classical oxidative and reductive degradation methods as a means of identifying some of the particular individual pyrrole ring substituents,3 a more insightful approach was taken by Budzikiewicz, et al. 4 in their observation that the hexahydro derivatives (porphyrinogens) underwent extensive cleavage under EI conditions to produce mono-, di-, and tripyrrolic fragments. Subsequently, Shaw, et al.5 in their study of alkyl substituted porphyrins obtained similar results via the porphyrinogens prouuced in situ in the mass spectrometer under H2 CI conditions, thereby avoiding the need to produce, handle, or isolate the fragile porphyrinogens themselves. We attempted to extend the H2 CI method via the desorption chemical ionization technique (D/CI) to porphyrins with vinyl, formyl, acyl, ethoxycarbonyl, cyano, and propionic ester substituents which have arisen during the course of other studies but found that their H2 D/CIMS were sufficiently cluttered with abundant secondary cleavage (benzylic) fragments as to make structural elucidation of the porphyrins difficult. Preliminary work indicates that much better results can be obtained through the use of the milder reagent gas ammonia since as shown (Figure 1) in the NH3 D/CIMS of 4-acetyldeuteroporphyrin IX DME (I_) extensive cleavage of the macrocycle occurs but without the excessive formation of secondary cleavage ions.