The objective of this article is to provide a description of moisture transport in buildingmaterials by application of a non-linear diffusion model, in which the effective coefficient of diffusion D-eff, being a function of moisture content, is implemented. This article proposes the method of determination of this coefficient through correlation of the theoretical desorption isotherms with the experimental ones. To this aim, a number of desorption isotherms at temperature T = 20 degrees C for several narrow ranges, being adjacent to each other, and for broad ranges of the air relative humidity Delta phi. were established experimentally for cement mortar samples with water/cement ratio w/c=0.50. The new knowledge contained in this article concerns some long-lasting measurements carried out in desiccators, for both narrow and broad ranges of the air humidity changes, and evaluation of the diffusion coefficient by applying an advanced optimization algorithm. The final conclusion is that the non-linear diffusion theory well reflects the moisture transport in hygroscopic porous materials like cement mortar.