Acetylene and oxygen gas plasma treatment of PAN-based carbon fibers to increase the interfacial shear strength was investigated. The effects of different gas mixtures and exposure times were studied. Changes in the fiber surface chemistry were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy, and wetting tests. The adhesion of epoxy to treated fibers was measured using the microbond technique and untreated fibers were tested to serve as controls. It was found that a significant improvement (alpha = 0.01) in the interfacial shear strength was obtained for fibers exposed to the plasma for 1 min at a ratio of 2 : 1 (acetylene/oxygen). However, the plasma treatments did not influence significantly (alpha = 0.01) the tensile strength of any of the fibers. FTIR analysis of the plasma film produced an glass slides under the treatment conditions showed that the expected chemical film composition for polyacetylene was achieved. Analysis of the fiber surface by wetting tests indicated a significantly (alpha = 0.01) increased advancing contact angle.