Although benzobisazole rigid-rod polymers are well recognized for their outstanding tensile and modulus properties as well as their excellent thermal and thermal-oxidative stabilities, they can only be fabricated from acidic solvents. Benzobisirnidazole polymers containing pendent benthiazole or sulfonic acid groups exhibit partial solubility in DMSO. Reaction of these systems with sodium methylsulfinylmethide in DMSO abstracts the acidic proton from the benzobisimidazo unit and forms the polyanion. Subsequent reaction of the polyanion with 1,3-propanesultone provides water soluble systems. Random copolymers containing benzobisimidazo and benzobisthiazole repeat units were prepared in polyphosphoric acid and subjected to the derivatization reaction. Benzobisimidazo content in the copolymers varied from 10–0 mole percent. Minimum ionic charge along the rigid-rod backbone to obtain water solubility was determined to be approximately 10 mole percent as reflected in the benzobisimidazole repeat units.