Oxygen plasma treatment was used to modify the surfaces of high-tenacity polyacrylonitrile-based carbon fibers. Changes in the surface composition of plasma-treated fibers were investigated by electrokinetic (zeta-potential) measurements as well as by wetting studies to evaluate the solid surface tension. Treating carbon fibers in an oxygen plasma results in an increased surface acidity, as can be seen from the overall shape of the zeta = f(pH) function and from the shift of the isoelectric points toward lower pH. The increased surface polarity improves the wetting behavior versus water. Although the fibers become more hydrophilic, the adsorption of ions from the electrolyte solution, which should compete with water adsorption, increases. Almost no changes in the surface morphology could be monitored by scanning electron microscopy. However, wetting and zeta-potential measurements confirm changes at the fiber surface. zeta-Potential measurements and wetting (contact angle) measurements complement each other. This allows observation of changes taking place in the surface chemistry after modifications applied to industrially available unmodified carbon fibers.