The minor aristolochic acids isolated from Aristolochia argentina were identified as 6,7dimethoxy, 6-hydroxy-7-methoxy, 2-hydroxy-8-methoxy and 7-hydroxy-8-methoxy disubstituted derivatives of the 3,4-methylenedioxy-IO-nitro-1-phenanthroic acid, respectively. A. argentina also contains the previously reported aristoloside. The mass spectra of the aristolochic acids, their esters and decarboxylation products have been examined. A number of successive fragmentation processes leading to the formation of aromatic hydrocarbons were observed. Cleavage of the nitro group is a prominent process in the mass spectra of the aristolochic acids and their esters. Evidence is presented that the formation of the [M - NO21 + ion occurs by an intramolecular aromatic substitution reaction with participation of the COIR group. The different behaviour of the decarboxylated aristolwhic acids is also discussed. A mechanism is proposed for the favourable loss of CH20 in the 8-methoxy isomer.