Platinum/graphite and tin/platinum/graphite catalysts have been characterized by cyclic voltammetry during liquid phase oxidations. A newly developed electrochemical cell allows the characterization of catalysts without transfer from the liquid phase to the gas phase. The platinum particle diameter, the degree of platinum coverage by carbonaceous species and the degree of platinum coverage by promoter metal were determined quantitatively during the oxidation of methyl-cr-D-glucopyranoside and ethanol. Reductive conditions can lead to the growth of the platinum particle diameter and to coverage by carbonaceous species. Hydrogen treatment at a pH of 13 leads to an increase of the platinum particle diameter from 2.7 nm to 7.4 nm and to complete coverage by carbonaceous species. After preparation of platinum/tin catalysts only twenty percent of the tin content of the catalyst is situated on the platinum surface. The degree of platinum coverage by tin can change during reaction. During the oxidation of methyl-alpha-D-glucopyranoside, deposition of tin from the graphite support on the platinum leads to an increase of the degree of platinum coverage by tin from 0.40 to 0.72.