An easy and sensitive electrochemical method for determining the number of carboxyl groups on carbon black (Vulcan XC 72R) was developed. This involves the modification of carboxyl groups on Vulcan XC 72R with an electrochemically active reagent (3,4-dihydroxybenzylamine) in the presence of a carbodiimide. The amount of reagent immobilized on a carboxyl group of Vulcan XC 72R was determined by cyclic voltammetry. Separation of the covalently bonded and physically adsorbed 3,4-dihydroxybenzylamine molecules is important. For this purpose, treatment with 3,4-dihydroxybenzylamine but without carbodiimide was performed, and the results were compared with those with the carbodiimide. This comparison can be used to assess the influence of the physical adsorption of 3,4-dihydroxylbenzylamine, and the number of molecules covalently bonded to carboxyl group can be determined quantitatively. Unoxidized and air-oxidized Vulcan XC 72R were evaluated with regard to the number of carboxyl groups per surface area. The effect of oxidation on the number of carboxyl groups is discussed.