Abstract The study of CO 2 electrochemical reduction to useful compounds using bare or modified BDD electrode attracts numerous attentions. Meanwhile, the efficiency of products obtained from CO 2 electrochemical reduction is known to be determined by the electrode material and the electrolyte. Formic acid as main product and CO as a minor product, have also been known on the CO 2 reduction using BDD electrode. Recently, we reported the successful improvement of CO production from the reduction of CO 2 by decorating the surface of BDD electrode with palladium particles. Following this, herein, we present further investigation on electrolyte dependence, including cation and anion dependence and also concentration effect in order to understand deeply the CO 2 reduction on surface of palladium modified BDD electrode. The results suggest the use of NaCl and KCl as a catholyte for optimum performance, in addition to the improvement of CO 2 reduction product in higher electrolyte concentration.