CO2 electrochemical reduction is of great interest not only for its technological implications but also for the scientific challenges it represents. How to suppress the kinetically favored hydrogen evolution in the presence of H2O, for instance, has attracted significant attention. Here we report a new way of achieving such a goal. Our strategy involves a unique water-in-salt electrolyte system, where the H2O concentration can be greatly suppressed due to the strong solvation of the high-concentration salt. More importantly, the water-in-salt electrolyte offers an opportunity to tune the H2O concentration for electrokinetic studies of CO2 reduction, a parameter of critical importance to the understanding of the detailed mechanisms but difficult to vary previously. Using Au as a model catalyst platform, we observed a zeroth-order dependence of the reaction rate on the H2O concentration, strongly suggesting that electron transfer, rather than concerted proton electron transfer, from the electrode to the adsorbed CO2 is the rate-determining step. The results shed new light on the mechanistic understanding of CO2 electrochemical reduction. Our approach is expected to be applicable to other catalyst systems, as well, which will offer a new dimension to mechanistic studies by tuning H2O concentrations.