Changes in surface physicochemical structures of polyacrylonitrile-based carbon fibers resulted from low current density electrochemical oxidation were monitored by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The relationship between the interlaminar shear strength (ILSS) values of carbon fiber-reinforced polymers (CFRPs) and carbon fiber surface chemistry including elemental ratios and the relative content of oxygen-containing functional groups were researched. SEM results revealed that the electrochemical oxidation got rid of surface contaminants generated during the production process. XPS analysis showed that the relative contents of oxygen and nitrogen increased by 446% and 202%, respectively, after the electrochemical oxidation. Carbon fiber surface chemistry was of paramount importance to the interfacial properties of CFRPs. The higher the carbon fiber surface activity, the better the interfacial bonding was, and an increase in the acidic-group contents was responsible for a higher ILSS value. However, when the current density increased to 1.0 A/m2, the interfacial bonding between carbon fiber and the epoxy resin became weak which led to the decline in ILSS values. Copyright © 2012 John Wiley & Sons, Ltd.