Poly(vinylidene fluoride) (PVDF) hollow fiber membranes were obtained by the phase inversion technique. The influence of internal coagulant viscosity (0.001 to 3 Pa s) and air gap (0.6 to 86.4 cm) on the structure and mechanical resistance of the fibers was studied. A “sponge-like” structure free of macrovoids was obtained by using polyvinyl alcohol (PVA) with N-methyl pyrrolidinone and water as internal coagulant (viscosity 3 Pa s). The effect of the air-gap was studied in order to control the structure and obtain mechanically resistant membranes with tensile strength at break between 2.2 and 54.3 N/mm and pure water permeability ranging from 4 to 199 Lhmbar. CO2 permeability of these membranes was measured and found to be in the range of 365 to 53200 NLhmbar. The “Dusty Gas” model (DGM) was used to calculate the pore size of the membranes from CO2 permeability experiments, obtaining pore radius values going from 0.6 to 10.8 µm. Results from modeling were compared with pore sizes observed in SEM images showing that this model can accurately predict pore radius of sponge-like structures; however, pore sizes of membranes presenting sponge-like structures together with finger-like pores were inaccurately predicted by the DGM.