Gas transfer through asymmetric polysulfone hollow fibre membranes has been modelled, allowing fine details of fibre structure to be deduced from gas permeation characteristics. The structural information is used to interpret the relationship between spinning conditions and fibre properties. Dope concentration determines the general morphology of the fibre, such as the porosity (voidage fraction), thickness of the active layer and order of magnitude of surface porosity (fraction of surface area that is pores), and thus it sets the permeability and level of selectivity that are likely to be achieved on coating. The selectivity of the solid polymer (the maximum selectivity achievable by any membrane if coating is highly effective or if no surface pores are present) was found to increase with increasing dope extrusion rate. The elevated levels of shear in the spinneret may enhance the orientation of polymer molecules. Increasing the jet stretch ratio during spinning had a detrimental effect on solid polymer selectivity. Increased elongational strain possibly results in an unfavourable polymer structure.