CO reduction studies over nanostructured copper catalysts are hindered by copper's instability in alkaline conditions, which results in dissolution during immersion into the electrolyte, leading to ill-defined catalyst morphologies and loadings. Immersing catalysts under potential control can alleviate this problem, but an experimental approach for cells generally used for CO reduction experiments is lacking. We demonstrate that by using an auxiliary electrochemical cell, electrodes can be introduced under potential control in these kinds of reactors. We investigated CO reduction over mass-selected copper nanoparticles using electrochemistry-mass spectrometry and show that the CO reduction activity increases by 4 orders of magnitude compared to experiments without potential control. This is attributed to the inhibition of Cu dissolution during immersion into the electrolyte as demonstrated by subsequent copper-stripping experiments. Thus, this study stresses the need for the application of such procedures in order to determine the intrinsic activity of nanostructured copper catalysts.