The mass spectral fragmentation pattern of a series of pyrido[3,2,1‐kl]phenothiazines is reported. The major fragments are derived from the breakdown of the pyrido ring and substituents thereon. The 1,2‐/2,3‐unsaturated, unsubstituted and most of the 3‐substituted compounds show the molecular ion as the base peak indicative of their relative stability toward electron impact. The genesis of the base peaks from the molecular ions of the 2,2‐dithiophenyl substituted compounds probably involves a concerted expulsion of a neutral diphenyldisulfide molecule and hydrogen atom transfer from the 1‐position to the 2‐position in the pyrido ring. On the other hand, 2‐thiophenyl substituted molecular ions lead to base peaks involving simultaneous ring opening of the pyrido ring, cleavage of phenylthiomethylene moiety as a radical and contractive ring closure to the pyrrole system. These two mechanisms appear to be diagnostic for distinguishing 2,3‐ versus 2,2‐positional isomers. The McLafferty rearrangement takes place in the 2‐thiophenyl and 2,2‐dithiophenyl substituted compounds. The retro‐Diels‐Alder fragmentation of the pyrido ring occurs in varying degrees depending on the nature of the 2‐ and 3‐ substituents and also the presence or absence of unsaturation in the pyrido ring. In the 2‐thiophenyl‐3‐keto compounds, thiophenyl group participation with 3‐carbonyl carbon and oxygen atoms is observed in the genesis of some interesting ion fragments. The detection of metastable ions in the spectra of 1,2‐dihydro‐2‐thiophenyl‐, 1,2‐dihydro‐2,2‐dithiophenyl‐3‐hydroxy‐, and 1,2‐dihydro‐2,2‐dithiophenyl‐3‐keto‐3H‐pyrido[3,2,1‐kl]phenothiazines; and spectra of 1,2‐dihydro‐3‐methyl‐, 1,2‐dihydro‐2‐carbethoxy‐3‐keto‐ and 1,2‐dihydro‐2‐thiophenyl‐3‐keto‐3H‐pyrido[3,2,1‐kl]pheno‐thiazines at low voltage support major fragmentation pathways.
