A new model has been developed to predict the permeability of plasma sprayed coatings, taking into account the dimensions and tortuosity of gas flow paths through the inter-connected porosity in such materials. The microstructure is assumed to incorporate large penny-like pores, located between splats, plus intra-splat micro-cracks and large equiaxed pores distributed uniformly throughout the coating. The behaviour is largely controlled by Knudsen flow through the fine inter- and intra-splat channels within the structure. The permeability is thus expected to depend on pressure, and to differ when measured at one atmosphere from the value expected at gas turbine operating pressures, which vary up to about 40 bar. Unfortunately, it is very difficult to perform these measurements at such high pressures. Experimental data1 for the gas permeability of plasma sprayed zirconia top coats, made at one atmosphere over a range of temperature, have been been compared with predictions from the model. Agreement is in general very good. Predictions from the model indicate that a significant increase in permeability is expected as the pressure is raised towards service levels, such that the partial pressure of oxygen at the interface between the bond coat and the top coat is expected to be almost the same as that at the free surface. This information is potentially useful when analysing the kinetics of bond coat oxidation. 1. Fox, A.C. and Clyne, T.W, Oxygen Transport by Gas Permeation through the Zirconia Layer in Plasma Sprayed Thermal Barrier Coatings, Surf. & Coat. Techn., 184 (2004) 311-321.