The reaction rate constant of SO3+NH3 in the gas phase was measured using a photofragment emission flow tube technique. The reaction rate constant at room temperature is (6.9¿1.5)¿10-11 cm3 molecule-1 s-1 at He pressures of 1--2 torr. This measured reaction rate is new information for studying the formation process of ammounium sulfates in the atmosphere. The reaction rate constant of SO3+CH3NH2, which is (1.1¿0.2)¿10-11 cm3 molecule-1 s-1, is also reported. ¿American Geophysical Union 1990