A quick, cheap, and accurate method for the determination of ammonia in air is described. Ammonia and water vapor are trapped simultaneously in a gas sampling tube cooled in liquid nitrogen. Subsequently ammonia is derivatized with o-phthaldialdehyde and determined using fluorescence detection. The detection limit of ammonia in a gaseous sample is about 1 nmol per liter of gas. The recovery, using a calibration gas of 6.00 ppm ammonia in nitrogen, is 102.9 ± 6.4%. Examples are presented in which this method is used for the determination of ammonia in environmental air and in expired air during exhaustive exercise of a human subject. It is suggested that this method can be used for the determination of volatile ammonia and other compounds in air during environmental and biological monitoring and in research. © 2001 Academic Press.