Composite materials can be enhanced by grafting a secondary material to a functional group on the surface of the reinforcing fibers to improve thermal, electrical or mechanical properties. Grafting secondary materials onto carbon fibers is often limited by the low reactivity of graphitic carbon and there is strong demand to create novel grafting methods with versatile functional groups. One desirable functional group is a carboxylic acid, which strongly interacts with many organic and inorganic materials. In this work, the surface of carbon fibers is functionalized by a reaction of naturally existing surface hydroxyl groups with isopropylidene malonate to graft terminal malonic esters, effectively creating a carboxyl functionalized surface. The reaction does not employ pre-oxidation to generate functional groups prior to grafting and is shown to preserve the tensile strength and morphology of the fiber. The surface functionalization is quantified by X-ray photoelectron spectroscopy, which shows that the relative surface coverage by carboxylic acid groups is increased from an initial 5.2% up to 9.2%. The effects of solvent, temperature, concentration and reaction time on the quantity of surface carboxylic acid groups are studied. This functionalization opens up new opportunities as a precursor reaction for further grafting reactions without sacrificing fiber strength.