A common way to describe moisture transport in materials is to include one term in the flux equation for “liquid flow”, that has moisture content as the transport potential. The transport coefficient, the diffusivity, is usually expressed as a monotonously increasing function of moisture content or RH. This is discussed in the paper and shown to be a crude over-simplification. Contradicting examples are taken from literature and comparisons are made with moisture profile measurements in two cases with temperature gradients in wood and in concrete.