The kinetics of the gas phase reaction SO3 + NH3 + N-2 reversible arrow H3NSO3 + N-2 were studied with a flow reactor coupled to a chemical ionization mass spectrometer for detection of SO3 and H3NSO3. The rate coefficient for the association reaction SO3 + NH3 + N-2 was measured as a function of temperature (280-340 K) and pressure (20-80 Torr of N-2). Analysis of the SO3 decay and H3NSO3 appearance at higher temperatures yielded rate coefficients for H3NSO3 decomposition and the enthalpy of the reaction SO3 + NH3 reversible arrow H3NSO3 : Delta H(298K)degrees = -24 +/- 1 kcal mol(-1).