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We have demonstrated detection of Cr(VI) ions using functionalized microcantilevers under electrochemical control. Au-coated microcantilever working electrodes are modified with self-assembled monolayers of 4-mercaptopyridine (4-mPy) in sulfuric acid solution. Differential surface stress changes at the modified microcantilever have been measured as a function of applied potential while simultaneously measuring the current-potential response by cyclic voltammetry. The selective adsorption of Cr(VI) on 4-mPy monolayer-coated microcantilever electrode is attributed to hydrogen bonding. The stressogram (first derivative of surface stress of microcantilever with respect to potential versus electrode potential) suggests that a small amount of an intermediate oxidation state of chromium is retained at the 4-mPy monolayer, and this intermediate reduction product of Cr(VI) can be reversibly oxidized on the surface. Our results demonstrate that changes of differential surface stress due to the adsorption-controlled electrochemical reaction can be observed at high sensitivity. This potential controlled microcantilever technique offers insights into the behavior of the solid-liquid interface during electrochemical reactions at modified electrodes.