Metallothionein (MT) is a low-molecular mass protein playing an essential role in homeostasis of heavy metals ions We used a chip-based CE for quantitative study of MT oxidation. After oxidation of MT by H2O2 we observed marked decrease in peaks heights and shift of peaks positions to higher molecular mass, which corresponded with the time of the oxidation. Moreover, we observed that the proportion of high-molecular forms of MT was markedly increased. The oxidative changes were successfully reversed by using of reducing agent prior to electrophoresis. Our method can reveal the stability MT aggregates, of which biological role still remains unclear.