Fabrication of a voltammetric sensor based on a self-assembled monolayer (SAM) of a heteroaromatic thiol, mercaptobenzimidazole (MBI) for the determination of uric acid (UA) in the presence of ascorbic acid (AA) is described. The MBI monolayer facilitates the oxidation processes of AA and UA. The orientation of the monolayer on the electrode surface controls the voltammetric behavior of AA and UA. The voltammetric signals of UA and AA are well separated with a potential difference of 270 mV and AA does not interfere with the measurement of UA at the monolayer-modified electrode. Linear calibration curves were obtained for UA over the concentration range 1-300 muM in neutral pH with a detection limit (3sigma) of 1 muM in the presence of 100-fold excess of AA. The practical utility of the present monolayer-modified electrode is demonstrated by measuring UA in human serum. The results obtained at the MBI monolayer are compared with those at the monolayers of structurally related aromatic thiols.