Special laboratory equipment has been developed to measure drying rates as a function of the product moisture content for Through-Drying of textiles, porous paper and other permeable products. This equipment allows very precise measurements even for very fast drying processes (1 second and less), which are typical in Through-Drying. Experimental results for the drying rate as well as for the permeability (pressure drop) as a function of the product moisture content for various drying conditions for porous paper (tissue) are presented. The analysis of these results reveals, that the drying rate during the ‘constant rate period'(surface evaporation period) can be predicted from standard heat and mass transfer equations only if the pore size distribution is taken into account. For the purpose of extrapolation and scale-up a special overall mass transfer equation has been introduced. For Through-Drying of tissue material, and of textiles, this equation represents the experimental results with fairly good accuracy.