Abstract The electrodeposition of Pt from aqueous solutions of K 2 PtCl 4 (Pt(II)), H 2 PtCl 6 (Pt(IV)), and a mixture of Pt(II) and Pt(IV) was studied using the electrochemical quartz crystal microbalance (EQCM) method. Pt deposition and cathode current flow began at the same potential in the Pt(II) solution. On the other hand, in the Pt(IV) solution, the cathode current increased at a more positive potential followed by Pt deposition at a more negative potential than in the Pt(II) solution. This difference in the potentials is due to the reduction reaction of Pt(IV) to Pt(II). Thus, Pt deposition in the Pt(IV) solution occurred in two potential ranges. In the first range, which was more positive than the second one, Pt was deposited via the reduction of Pt(II) to Pt(0). In the second range, direct deposition from Pt(IV) to Pt(0) proceeded, but was followed by hydrogen adsorption, which inhibited further Pt deposition.