Effects of impurities such as NO2 and SO2 with or without co-presence of O2 were studied in electrochemical reduction of CO2 on Cu in 0.5 M KHCO3 or 2 M KCl electrolyte in LS (liquid-solid) or GLS (gas-liquid-solid) cell. The effects of gaseous impurities are complex depending on geometry, flow rate, and speciation in solution. Generally, less than or equal to 1667 ppm of exposure NO2 is either beneficial or neutral - dissolved nitrate is beneficial, but nitrite and ammonia are detrimental. Less than or equal to 104 ppm of exposure of SO2 is either mildly detrimental or neutral - dissolved sulfide is extremely detrimental, but sulfate is beneficial. Oxygen can be detrimental (co-reductant or catalyst oxidant) or beneficial (oxidation of dissolved sulfide species) depending on LS or GLS geometry.