By using differential optical absorption technique combined with Fouriertransformation we achieve a good separation of ammonia from sulfur dioxide inthe wavelength range 207 nm to 217 nm. Measurements shows that with apathlength of two decimeters and a measuring time of one minute we canmeasure less than 1.32 mg/m3 {1.9 ppm(volume)] of ammonia with a standarddeviation of 0.46 mg/m3 [0.66 ppm(volume)] with interference from 408.73mg/m3 [156 ppm(volume)] sulfur dioxide.