Permeability is widely considered as a measure of durability. The main objective of this study was to investigate some of the significant factors that influence the measured relative gas permeability of concrete. In particular the study set out to reduce the time taken to perform a permeability test. Initially the effect of conditioning at 105°C and 85°C on the strength and permeability of Normal Strength Concrete (NSC) and High Strength Concrete was examined. It was shown that although similar compressive and tensile strength results were obtained irrespective of conditioning temperature, marginally higher permeabilities were reported for the concrete conditioned at 105°C. Concrete specimens were then conditioned to obtain varying degrees of percentage weight loss to determine whether conditioning to a completely dry state could be avoided and whether reliable results could be achieved from concrete specimens similar to those found in-situ. A power relationship resulted between percentage weight loss and relative permeability; therefore, it is possible to estimate the permeability of a specimen from the test results without the need to condition it fully. Next, a study into the effect of gas path length on the relative permeability of NSC and a higher strength concrete was performed. From the study a linear relationship was observed between the two variables for both concretes signifying that smaller specimens can be tested and reliable permeability results obtained. However, coefficients of variation of between 30% and 45% have been noted repeatedly for relative gas permeability results from the experimental investigations. Finally, an analytical and a finite difference numerical model were developed to replicate the relative permeability test set up. Both of the models showed excellent agreement with the experimental data and therefore relative permeability test results can be used to obtain intrinsic permeability coefficients. The benefit of the models is that within the limits of the relative permeability test set up intrinsic permeability coefficients can be obtained for specimens of any diameter and length.