The stability of benzobisoxazole and benzobisthiazole compounds and polymers under hydrolytic conditions was studied. 2,6-Bis(4-tert-butylphenyl)benzo[1,2-d; 4,5-d']bisoxazole (1) dissolved in acetonitrile containing sulfuric acid and water at 80 degrees C is stable. A suspension of 2,6-bis [4-(2-benzoxazoyl)phenyl] benzo [1,2-d;5,4-d'] bisoxazole (2) in 0.2 N H2SO4 or 0.2 N NaOH solution at 100 degrees C for 21 days is stable. The intrinsic viscosity of a poly(p-phenylene)benzobisoxazole (PBO) fiber sample soaked in 0.2 N H2SO4, water with 1 wt % polyphosphoric acid (PPA), or 0.2 N NaOH remained the same. Under very severe hydrolytic conditions such as dissolution of compound 2 or PBO in PPA or methanesulfonic acid with residual water followed by coagulation in water, benzobisoxazole underwent bond cleavage to generate carboxylic acid and o-aminophenol functional groups. This is in contrast to an earlier hypothesis that the decrease in intrinsic viscosity under these conditions was due to chain association. Poly(p-phenylene)benzobisthiazole (PBT) also underwent bond cleavage under these very severe conditions, which are unlikely to be encountered in normal applications.