Abstract The mass transfer of NH 3, into water at environmental concentrations has been studied in a two-phase flow reactor. The concentration of NH 3 in water ranged from 10 −6 to 10 −4 mole l. −1 and the temperature was chjanged from 12 to 30°C. At the conditions of this experiment, the overall mass transfer coefficient ( K) is more dependent on the hydrodynamics of the gas phase than of the liquid phase. No significant concentration or temperature effects on the value of K have been observed. The mass transport of NH 3 is interpreted in terms of the individual gas phase and liquid phase mass transfer coefficients.