Time Domain Reflectometry (TDR) measurements revealed an abrupt change in the slope of the relationships between dielectric constant (DC) and water content (theta) for soils and sand-kaolin mixtures of heavy texture. The calibration curve DC (theta) can be satisfactorily approximated by a model that is based on the assumption of the influence of bound and film waters with averaged effective DC lying in the range from 16 to 28, on the shape of this curve at a low water content.