The electrochemical behavior of a copper(I)oxide (Cu2O) modified carbon paste electrode (MCPE) was investigated in different buffer solutions and in the presence of ascorbic acid (AA) and cysteine (RS). Working conditions such as pH, mineral oil, and modifier ratio were optimized. Voltammetric results revealed that RS forms rather stable complexes with Cu(I) which has a high electrocatalytic reduction peak current at -0.65 V versus SCE in borate buffer (pH 9.2). In the case of AA, a complexation occurred with Cu(II) species at the electrode surface, rather than Cu(I). The electrocatalytic reduction peak current of the Cu(II)-AA complex was observed at -0.07 V in phosphate buffer at pH 6.9. Linear responses were observed in the range 2.0 x 10(-9) to 3.0 x 10(-8) M with a 0.9954 correlation coefficient for RS and 1.0 x 10(-9) to 2.0 x 10(-8) M with a 0.9961 correlation coefficient for AA. (C) 2004 Elsevier B.V. All rights reserved.