We have developed a technique to determine the permeability of paper structures from X Ray microtomographic images. The images are thresholded and a skeleton of the pore or fiber spaces is obtained from the images. The skeleton is represented by means of a network of resistances. The total resistance of the network in the different dimensions is now determined both in the in-plane as well as through plane directions. The network resistance for structures at each section is determined. From this data, a correlation length for the network resistance is obtained. Correlation length data are used for determining the effect of heterogeneities on transport properties of paper structures.