As a starting point, cuprous oxide catalysts were synthesized via a simple room-temperature route (Fig. ). A catalyst ink from these materials was loaded onto a standard gas diffusion layer and was used as the working electrode in a modified gas-diffusion half-cell configuration (Fig. ). In this setup, CO2 was fed through the gas phase directly to the gas/catalyst/electrolyte interface, thereby promoting CO2 reduction by circumventing its solubility limits and enabling the use of alkaline electrolyte (1 M KOH) which suppressed the hydrogen evolution reaction (HER). 200 mM SO3 2-as the sulfur source was added directly to the aqueous electrolyte. Screening of possible products in the NMR spectra revealed that HMS, SA and MS were formed as 6e -, 8e -and 8e -products (Fig. and S8, 9). While enabling new reaction pathways, the presence of sulfite decreased the total current density (Fig. ) and favored the production of liquid products in from CO2 reduction (Fig. ). Across the measured potential range, the FE for detected C-S products ranged from 4.7-9.5% (Fig. ) and the partial current density for each only modestly increased with more negative potentials (Fig. ). This weak dependence on product formation with applied potential potentially indicates that a chemical, rather than electrochemical step may be rate-determining. The production of HMS, SA and MS from CO2 and SO3 2-represents three new reaction pathways and opens up the possibility of building up a wide gamut of thiols, sulfonates, organic sulfides and more with simple electrochemical tools as an alternative to currently employed organic chemistry routes. HMS has been observed before from the nucleophilic attack of CH2O (locally generated from CO) by SO3 2-and in our case a similar coupling step may be in play. SA and MS on the other hand, is tentatively thought to arise from a (near)surface SO3 nucleophilic attack onto a C2 intermediate in the methane or acetate pathway. There was no HMS, SA or MS formed in control experiments that omitted CO2, SO3 2-, the Cu catalyst or an applied bias (Fig. ). Further, using commercially purchased Cu, Cu2O, CuO or synthesized Cu2O with other morphologies resulted in HMS and SA formation with lower, but comparable FEs (Fig. , 16), indicating that the C-S bond pathway is generalizable on a variety of Cubased materials. In contrast, Ag particles which produce CO as the main CO2 reduction product, did not show any detectable C-S products (Fig. ), indicating the need of the catalyst to produce and stabilize further reduced intermediates than CO. When SO3 2-was omitted from the electrolyte, the main difference in the product distribution was that methane was a dominant product (Fig. ). This may indicate that the route to C-S products shares a similar pathway to methane formation and thus intercepts intermediate before they are reduced to methane or that the presence of near-surface SO3 2-alters the reaction route to minimize the methane pathway. We next moved to carry out a round of experimental mechanistic studies to gain an initial layer of insight into the dynamics of the electrosynthetic system. The faceted Cu2O starting material loses its well-defined structure during electrocatalysis under reductive potentials, as evident from a comparison of transmission electron microscope (TEM) images (Fig. ). Translating the setup to a powder x-ray diffraction (XRD) instrument enabled us to visualize changes in crystallinity immediately after applying a reducing potential. Indeed, the Cu2O peaks disappear within 4 minutes after shifting from open circuit conditions to -0.68VRHE and only Cu peak stemming from metallic Cu remains (Fig. ). This indicates that while an oxide is the starting material, the main phase of the catalyst under working conditions is indeed metallic Cu, though sub-surface oxygen or defects induced by the structural change could certainly be present and affecting the catalytic process.