The adsorption of vitamin Ki at a hanging mercury drop electrode at pH 4.2 allows the determination of this compound in the 1 × 10−6M−1 × 10−9Mconcentration range. Applying a square-wave potential scan provides a 200-fold increase of the peak current compared to differential pulse stripping voltammetry. The influence of several operational parameters such as the nature and the pH of the supporting electrolyte, the deposition time and the potential are discussed. Interactions between the adsorbed molecules occur during the deposition step.