An electrochemical investigation of free thiamin and its phosphate esters was performed in acidic solutions at an unmodified platinum electrode by means of cyclic voltammetry. The results obtained provide information that reflects the redox properties of thiamin itself rather than its usual interaction with mercury from polarographic analyses. In acidic media, the protonated thiamin (A(2+)) at the N1' position in the pyrimidine ring was found to be the electroactive species. A(2+) was reduced in a one-electron transfer mechanism to produce a very reactive radical cation, A(-+) which underwent a fast dimerization [AA](2+). In the reverse scan, the dimer [AA](2+) was oxidized back to the starting material (A(2+)) in a two-electron transfer process. Both pH and electrode materials played an important role in the determination of the proposed mechanism. (C) 2004 Elsevier B.V. All rights reserved.