The fast atom bombardment mass spectral observations on some P-halogenated tetraphenylporphyrins and their iron derivatives were compared in order to investigate the relative stability of these complexes, The investigated compounds are represented as O[FeBr4TPP](2), O(FeTPP)(2), O[FeTPP(4-Br)](2), O[FeTPP(4-OCH3)](2), H-2(Br4TPP), Fe(Br4TPP)Cl, H-2(Cl16TDCDMPP) and Fe(Cl16TDCDMPP)Cl where Br4TPP is the dianion of 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin, TPP is the dianion of 5,10,15,20-tetraphenylporphyrin, TPP(4-Br) is the dianion of 5,10,15,20-tetra(4-bromo)phenylporphyrin, TPP(4-OCH3) is the dianion of 5,10,15,20-tetra-(4-methoxy)-phenylporphyrin and Cl16TDCDMPP is the dianion of 2,3,7,8,12,13,17,18-octachloro,5,10,15,2 tetra-(2,6-dimethoxy,3,5-dichloro) phenylporphyrin, The stability of each molecular ion and the subsequent fragmentations were correlated to the substitution of the porphyrin rings and to the loss of steric hindrance.