The effect of size on carbon fiber surfaces was examined by x-ray photoelectron spectroscopy and scanning electron and x-ray wavelength dispersive microscopies. The fibers studied had been subjected to various levels of commercial surface treatment before the commercially applied size was added. The removal of the size from the fiber surface was examined both by using solvents and by heating the sized fibers. The surface chemistry of the sized fibers was dominated by the sizing and indicated that more sizing was often found on the fibers that received greater prior surface treatment. The study suggests that some significant interaction occurs between the size and the surface treated fiber surface, and this interaction leads to a greater stability of the size on the fiber surface.