We report on the surface characterization of commercial poly(p-phenylene benzobisoxazole) (PBO) fibers. Several solvents (hexane, acetone, and ethanol) were employed to remove the sizing present on the fiber surfaces. The surface properties of the different samples were studied by means of inverse gas chromatography (IGC) at infinite dilution using nonpolar n-alkanes and molecules with different acid-base characteristics (benzene, pyridine, acetonitrile, nitromethane, tert-butyl alcohol). Complementary information was obtained by using thermogravimetric/differential thermal analysis (TG/DTA) and atomic force microscopy (AFM). Results showed that, whereas the sizing of high-modulus (HM) fibers can be gradually washed off as the polarity of the solvent increases, only ethanol washing allowed to detect changes in the surface characteristics of PBO as-spun (AS) fibers, with hexane and acetone having little or no effect. In fact, it is believed that the effect of any coating applied eventually to the PBO AS fibers is masked by the presence of poly(phosphoric acid) (PPA) or partially coagulated PPA/PBO residuals on their surfaces, which in turn determine their surface properties. In any case, both standard finishes and contaminants strongly decrease the surface energy of the PBO fibers. According to the experimental results, it was assumed that PBO pristine fiber surfaces could be obtained by ethanol washing. Further differences were found between the cleaned AS and HM fibers. AFM measurements showed microfibrils present in the HM fibers that are somewhat wider than their AS counterparts. The presence of voids at the surface level of the cleaned PBO AS fibers brought about strong changes in adsorption energetics. No such changes were detected in the case of cleaned HM fibers. The contaminants of the AS fibers confer them a more acidic character than that of the rest of samples studied. Sizing/contaminants removal on both fiber types leads to an increase in the number or strength of basic sites. However, the cleaned fibers kept an amphoteric character based on the strong specific interactions existing between pyridine and the fiber surfaces. Ab initio calculations carried out on an appropriate model confirmed the presence of positively charged carbon atoms in the PBO monomers which should act as electron acceptors. Moreover, PBO fibers are prone to exert important pi-pi interactions as it was found both experimentally (relatively high -DeltaH(a)(SP) values of benzene) and predicted from the charge distribution obtained from theoretical a calculations.