Process for preparing bisimides and bisimides prepared thereby

White N,N'-alkylene-bis-tetrabromophthalimides are prepared by reacting about two molar proportions of tetrabromophthalic anhydride with about one molar proportion of a diaminoalkane containing 2-6 carbon atoms in a solvent mixture having a boiling point of at least about 125 DEG C. and comprising about 50-97.5% by weight of a liquid aromatic hydrocarbon or halohydrocarbon having a boiling point of at least about 80 DEG C. and about 50-2.5% by weight of an alkanoic or aralkanoic acid having a dissociation constant not higher than about 6 x 10-5 at 25 DEG C. A preferred solvent is a mixture of about 70% by weight of xylenes and about 30% by weight of propionic acid having a boiling point of about 130 DEG C. The products are particularly useful as flame retardants.