The characters of poly(p-phenylene benzobisoxazole) (PBO) in methanesulfonic acid (MSA) are strongly influenced by the protonation effect. Due to such protonation effect, the experimental optical absorption spectrum was red shifted from the optical absorption spectrum predicted by AM1/ZINDO-CI (Austin Method 1/Zerner's intermediate neglect of differential overlap coupled to single configuration interaction) method. Further studies on AM1-optimized geometries of PBO showed that the final minimized torsion angle of benzoxazole and phenylene rings increased from 0.04degrees for the neutral molecule to 18.53degrees for protonated form at N atom. Moreover, the repulsive Coulmbic interactions originated from protonation tend to stiffen the PBO chain. The AM1/ZINDO-CI calculation also indicated that the strongest UV absorption peak of PBO was blue shifted with increasing torsion angle. (C) 2004 Elsevier Ltd. All rights reserved.