The integration of organic adlayers with diamond surfaces imparts many new properties to diamond materials, including lubrication, optical response, chemical sensing, and biocompatibility. The development of new immobilization chemistries is required in which organic layers are covalently attached to the diamond surface. Poly(methyl methacrylate) and polystyrene brushes (PMMA, PS, respectively) were grown from the surfaces of ultrananocrystalline diamond (UNCD) thin films that were modified by the electrochemical reduction of the diazonium salt BF4 ~,+N2– C6H4–CH(CH3)–Br. The grafted –C6H4–CH(CH3)–Br species were effective in promoting surface initiated atom transfer radical polymerization (SI-ATRP) of the vinyl monomers. Attempts to perform SI-ATRP from undoped diamond films produced by microwave plasma assisted chemical vapour deposition (MPACVD) process was rather unsuccesful due to a very low extent of electrochemical modification by the brominated aryl diazonium salts. The formation of the polymer brushes was confirmed by XPS. The polymer brushes were further evaluated as adsorbents for human serum albumin (HSA) used as a model protein. Protein adsorption decreased in the order PS>PMMA, that is the decreasing trend of hydrophobic character.