Air compressibility is important in attaining an accurate quantification of the permeability parameters for refractory castables, Nevertheless, this effect is often neglected in the permeability equations that are found in ceramics literature. The consequences of omitting the compressibility effect in two of the main permeability equations are highlighted and discussed. The permeability to air flow of high-Al2O3 ultra-low-cement refractory castables has been quantified by Darcy's law according to U,S, ASTM C577-96 and European PRE-16th permeability methods, which propose distinct approaches for air compressibility. Forchheimer's equation, which establishes a more realistic dependence between fluid pressure and fluid velocity, also has been used to obtain the permeability constants k(1) and k(2), Results show that equations proposed by the ASTM and the PRE methods may yield very distinct values of Darcian permeability k(1), for the same sample. Forchheimer's equation for compressible fluids provides the best fitting with experimental data.