Benzo[ b]naphtho[2,3- d]thiophene-6,7-dicarboxylic anhydride ( I) is obtained y Pschorr intramolecular cyclisation of 2-(2-aminophenylthio)-1,8-naphthalic anhydride. Condensation with alkylamines and arylamines gives the corresponding imides, which colour synthetic-polymer fibres in greenish-yellow hues, and reaction with o-phenylenediamines affords the isomer mixtures of derivatives of benzimidazo[1,2- b]-dibenzothiopheno-[4,3,2-d, e] isoquinolin-10-one and of benzimidazo-[2,1- a] dibenzothiopheno-[4,3,2-d, e]isoqumolin-10-one, which dye polyester fibres bright reddish-orange. Both series of dyes have excellent fastness to light and sublimation. The colour of the dyes is discussed in relation to the isomeric compounds derived from benzo[ k,1]thioxanthene-3,4-dicarboxylic anhydride ( III) and from benzo[d]naphtho[1,2- b]thiophene-9,10-dicarboxylic anhydride ( II). The synthesis of the individual isomers resulting from reaction of I with o-phenylenediamine is reported and their absorption maxima related to polar factors operative within each isomer.