Platinum (Pt) dissolution has been elucidated by measuring a mass loss of Pt black-deposited Au quartz resonators in 0.1 mol dm-3 HClO 4 containing NaNO2. A continuous weight loss of ca. 50 ng cm-2 cycle-1 was detected at [NaNO2] = 0.01 mol dm-3 under the potential cycling between 0.6 and 1.4 V vs. RHE. The magnitude of Pt dissolution depends on the concentration of NaNO2 and the range of potential cycling. The average mass loss decreases to 5∼10 ng cm-2 cycle-1 at [NaNO2] = 1 mmol dm -3. The Pt dissolution occurs when the higher potential limit is more than 1.0 V vs. RHE and the lower potential limit is less than 0.9 V vs. RHE, suggesting that the reduction of Pt oxide in the presence of NaNO2 in the bulk solution contributes to the dissolution.