Abstract: A method for the determination of ammonia in ambient air and exhaust gas by gas phase molecular absorption spectrometry was proposed. When collecting ambient air sample, 25 mL of 0.01 mol·L^-1 sulfuric acid solution as absorption solution was added into 25 mL-absorption tube, and then sampling was conducted for 0.5-1.0 h at flow rate of 1.5 L·min^-1. When collecting exhaust gas sample, 50 mL of 0.01 mol·L^-1 sulfuric acid solution as absorption solution was added into 50 mL-absorption tube, and then sampling was conducted for 0.5 h at flow rate of 0.5 L·min^-1. The detection wavelength of gas phase molecular absorption spectrometer was set as 214.7 nm (deuterium light source) or 213.9 nm (zinc hollow cathode light source), and the heating temperature was set in the range of 80-90 ℃. It was confirmed that Mn²⁺, aniline, ascorbic acid, Mn⁷⁺ and ClO^- would interfere on the determination of ammonia through interference test, and the interference substances above could be eliminated by oxidation, reduction, distillation and dilution methods. The series of ammonia standard solution with mass concentration no more than 14.0 mg·L^-1 was used as the mother liquor to draw the calibration curve. It was shown that the linear range of the ammonia calibration curve was within 4.00 mg·L^-1, with detection limits of 0.008 mg·m^-3 (ambient air) and 0.03 mg·m^-3 (exhaust gas). Test for recovery was made on the actual samples of ambient air and exhaust gas at 3 concentration levels by standard addition method, respectively, and the recoveries were in the range of 97.9%-106%, with RSDs (n=6) of the ammonia determined values less than 2.5%. Meanwhile, there was no significant difference between this method and the current effective Nessler's reagent spectrophotometry by F-test using paired samples.