For the predictive modelling of a membrane distillation process, the gas transport properties, defined by the dusty-gas model, of three highly permeable polyethylene and polypropylene fibre membranes have been determined. Single gas permeation experiments were carried out to determine the Knudsen diffusion and viscous flow membrane characteristics (K0 and B0, respectively). Binary gas diffusion experiments were carried out to determine the molecular diffusion membrane characteristic (K1). Because of the high permeability of the fibre membranes, new methods were developed to deal with effects such as pressure drop in the single gas permeation experiments and boundary layer resistance in the binary gas diffusion experiments. The K1 values of the fibre membranes were determined with an inaccuracy of 4–8%. It turned out that calculations of K1 with the values of K0 and B0 assuming cylindrical pores are, for the membranes studied, inaccurate by a factor of two.