This paper reports the successful preparation of catalytic electrodes based on carbon-fiber paper modified with dendrimer-encapsulated platinum nanoparticles. The metallic nanoparticles were first synthesized from solution within generation-four hydroxyl-terminated PAMAM dendrimers, which serve as a carrier for their subsequent immobilization on the solid substrates. The carbon-fiber surfaces were activated by means of three alternative anodic pretreatments and then loaded with the dendrimer-metal nanocomposites by cycling of the carbon-electrode potential. The degree of oxidation of the carbon surface affects the anchoring of the dendritic material, the coverage of which is indicated by the electroactive area of the encapsulated platinum. The modified carbon-fiber surfaces pretreated by cyclic polarization are found to be electrocatalytic for the oxygen reduction reaction, presenting a good exchange-current density at low platinum loading.