Based on Kohsiek's fast air circulation chamber, a method has been developed to measure the surface resistance to vapor diffusion in a drying topsoil. This resistance is important to estimate evaporation from bare soils using an aerodynamic resistance formulation. Measurements were done for a fine sandy loam during a dry down after artificial wetting. Surface resistance started to increase at a moisture content of 15% by volume in the 1-cm top layer, which is 50% of its moisture content at field capacity. Calculations of the aerodynamic resistance were corrected for stability and were used to isolate the real surface resistance from the bulk resistance. Resistances could be modeled as a function of the top 1 cm soil moisture and varied between approximately 10 s/m for a wet and several thousand seconds per meter for a dry top layer. The measurements demonstrated a very pronounced diurnal course due to drying of the very top layer during the day and recovery of the moisture profile during nighttime hours.