The wind-tunnel technique is widely used to measure ammonia emissions from extensive surfaces. However, few studies have been made, within the tunnel, of the turbulent flow which drives the transfer processes above the experimental surface. Experiments have been carried out to determine the transfer characteristics within the tunnel, and the influence of the external wind on these characteristics. Two source types have been used, one with constant surface emission of CO2, and one with constant surface concentration of water vapour. Both water vapour emission rate and wind speed measurements show the building up of a boundary layer above the experimental surface, which was dramatically disturbed by the external wind in the first metre. Further downwind, the boundary layer stabilizes to a height of about 10 cm, below which the characteristics were equivalent to those of the atmospheric boundary layer, as shown by CO2 and wind speed measurements. However, above the boundary layer, the wind speed in the tunnel was consistently lower than the external wind speed, showing that if the wind speed at 25 cm is used to drive the tunnel flow rate, the transfer rate in the end part of the tunnel would be consistently higher than outside.