The emissivity of soil surfaces at microwave frequencies depends strongly on the near-surface water content. However this relationship varies between soils. Use is made of a model (MICRO-SWEAT) which couples water, heat and microwave radiation transfers through soils to show that the main cause of soil variation in the relationship between apparent emissivity at 1.4 GHz (eapp) and the average water content of the upper 2 cm of the profile (θ0-2) are differences in the dielectric properties. A simple semi-empirical model is proposed, based on the effect of soil particle size distribution on bound/free water partitioning, which provides a calibration between eappand θ0-2. Although, in principle, the detailed shape of the water content distribution close to the surface will influence the eapp/θ0-2relationship, such effects are only significant during short periods (< 1 h) following the onset of rainfall which produces steep wetting fronts close to the soil surface.