An attempt was made to remove by-products such as NH3, CH3CN, HCN, CO and CH3CHO which are produced from the reduction of lean NOx by ethanol over an alumina-supported silver catalyst. The efficiency of titania-supported noble metals, vanadium oxide, tungsten oxide, and copper sulfate for the removal of these compounds was examined. It was found that NH3, CH3CN, and HCN were easily oxidized to NOx and that the reduction of NOx to N-2 by these nitrogen-compounds did not occur effectively on noble metal catalysts. Among the tested catalysts, CuSO4/TiO2 was the most effective for simultaneously removing NH3, CH3CN, and HCN and reducing NOx to N-2 by using these nitrogen-compounds. Lean NOx were reduced to N-2 by ethanol effectively and the concentration of nitrogen-containing by-products was lowered over the composite catalyst, Ag/Al2O3 + CuSO4/TiO2. However, in addition to CuSO4/TiO2, Pt/TiO2 was required to remove harmful compounds completely. Consequently, a three-component composite catalyst, Ag/Al2O3 + CuSO4/TiO2 + Pt/TiO2, has been employed in reducing lean NOx by ethanol. This composite catalyst has proved to be quite effective even in the presence of water vapor in that it can reduce lean NOx to N-2 by ethanol effectively and sufficiently lower the concentrations of all harmful by-products.