This work describes the use of a new permeametry technique to evaluate microstructural changes that take place in castables with high alumina-fiber content during the dewatering process. It is shown that the mass loss due to cement dehydration and the permeability increase caused by fiber burnout can be independently assessed, based on the fluid dynamic conditions established for the experiment. Control of the pressure drop through the sample was the key procedure used to analyze the phenomenon of interest.