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A microfluidic-enabled electrochemical device has been developed to investigate electrochemically active nanomaterials under reaction conditions using in situ scanning transmission soft X-ray microscopy (STXM). In situ STXM measurements were conducted on electrodeposited Cu catalysts under electrochemical CO 2 reduction (CO 2 R) conditions. The study provides detailed, quantitative results about the changes in the morphology and chemical structure of the catalytic nanoparticles as a function of applied potentials. The deposited Cu nanoparticles initially contain both Cu(0) and Cu(I). As an increasingly cathodic potential is applied, the Cu(I) species gradually convert to Cu(0) over the potential range of +0.4 to 0 V versus the reversible hydrogen electrode (V RHE ). During this process, Cu(I) particles of various sizes are converted to metallic Cu at different reaction rates and at slightly different potentials, indicating a degree of heterogeneity in the electrochemical response of discrete particles. At CO 2 R relevant potentials, only metallic Cu is observed, and the morphology of the particles is fairly stable within the spatial resolution limits of STXM (∼40 nm). We also report in situ STXM studies of a working electrode with relatively thick Cu-based electrodeposits. The spatially resolved chemical analysis identifies that Cu-oxide species can persist under CO 2 R conditions, but only when the catalytic nanoparticles are electronically isolated from the working electrode and therefore are catalytically irrelevant. In summary, in situ STXM is presented as a technique to gain advanced morphological and spatially resolved chemical structure insights into electrochemically active nanomaterials, which was used to provide improved understanding regarding Cu nanomaterial catalysts under CO 2 reduction conditions.