the conventional condensation of naphthalene - 1 , 4 , 5 , 8 - tetracarboxylic acid with 1 , 2 - diaminobenzene gives the resulting naphthoylenebisbenzimidazole isomers together in the cis and trans form . it is desirable to shift the isomer ratio of the mixture obtained toward the desirable trans product and to reduce the amount of cis product obtained . it is also desirable to develop a process of preparation which optimizes the yield and improves the waste water situation . this object is achieved according to the invention by subjecting virtually stoichiometric amounts of the starting compounds to a precondensation at 60 ° to 110 ° c . in an aqueous medium and then completing the condensation in the aqueous medium in a second stage at 130 ° to 180 ° c .