Carbon fibers are oxidized in two electrolytes at different electrolyte concentrations and potentials. The chemical composition of the fiber surface changes considerably in both cases. The oxidation in H2SO4 results in the formation of sulfur containing groups, but quinoidal compounds are also detected on the surface. The concentration of all functional groups increases with increasing electrolyte concentration at 5V, but does not change as a function of oxidation potential in 20 wt% solutions below this value. Carboxylic functional groups are formed on the fiber in NaOH, but some adsorbed NaOH also remains on the surface after oxidation. Cyclic voltammetry reflects the modification of the surface differently. Peak potentials remain basically constant, while peak currents depend on the type of the treatment. Good correlation has been found among peak current, the chemical composition of the fiber surface and IFSS of epoxy composites in the case of H2SO4. The adsorbed NaOH interferes with the electrochemical reaction that takes place on the fiber surface. Moreover, IFSS decreases as the amount of adsorbed NaOH increases.