The electron impact mass spectra of some benzo[1,2-b:4,5-b']dithiophene-2,6-dicarboxylic acid dianilides and dithieno[3,2-b:2',3'-d]thiophene-2,6-dicarboxylic acid dianilides are discussed. Dominant peaks in these dianilides are formed by the cleavage of a C-N bond on one side of an anilino group. These ions fragment further by the cleavage of a C-C bond on the other side of an anilino group and a CONRPhR' group may be lost directly. After loss of CO, the characteristic benzodithiophene radical cation, C10H2S2Cl2[symbol: see text], at m/z 256 and the dithienothiophene radical cation, C8S3Cl2[symbol: see text], at m/z 262 are formed from their respective precursor compounds.