The absorption of lean NOx (200 similar to 1000 ppm) gases in aqueous P-4 emulsions was investigated in a mechanically stirred vessel. The chemical reaction rate between NOx and P-4 in an alkaline solution was shown zero order with respect to NO concentration and three-seconds order to P-4 concentration. In the presence of SO2, the chemical reaction rate between NOx and P-4 in alkaline solution was again zero order with respect to NO concentration, but changed to second order to P-4 concentration. In addition, a robust design method was also used to evaluate the relative importance of various processing variables on the performance of this deNO(x) process by P-4 solution. S/N analysis of the results indicated that the P-4 concentration, liquor temperature and NOx concentration were the major factors in affecting NOx absorption efficiencies.