Considerable efforts have been made to understand the dewatering behavior of refractory castables. Modeling of this process must be based on realistic heat and mass transfer data; hence, consistent values of castable properties are required, particularly permeability values, since these are the most important parameters that govern the process. Permeability values, however, are usually obtained at room temperature and therefore may not reflect the remarkable microstructural changes that take place during water removal. The problem has become more critical for castable compositions based on calcium-free binders, in which the dehydration process is still unclear and explosive spalling is more likely to occur. This study has investigated the behavior of the dynamic permeability of high-alumina calcium-free refractory castables subjected to a drying process up to 700°C. Samples were previously treated at temperatures ranging from 110° to 1650°C to provide information on the castables’ reversible and irreversible microstructural changes. The results revealed fluctuations and a dramatic decrease in the permeability level near 200°C, which may help to explain the occurrence of explosive spalling in this class of castables.