Condensation of o‐phenylenediamine with 2‐, 3‐ and 4‐nitronaphthalene‐1,8‐dicarboxylic anhydrides gives, in each case, an isomer mixture. Separation of the mixtures into the six isomeric mononitro‐7H‐benzimidazo[2,1‐a]benz[de]isoquinolin‐7‐ones is described and the isomers characterised in respect of previously reported data on some isomers; ir and mass spectrum data are reported, the latter showing fragmentation ions which may be used in characterising dyes based on this chromophore. Other new derivatives of 7H‐benzimidazo[2,1‐a]benz[de]isoquinolin‐7‐one are also described.