Time-lag method is a very popular technique which has been developed and widely used for the measurement of diffusion coefficients on polymer membranes. In this work we applied the transient time-lag method for the determination and the modeling of a global diffusion coefficients for CO2 and CA through a composite MFI cylindrical membrane. The developed equations were used for the estimation of global apparent diffusion coefficients which includes the Knudsen like diffusion through intercrystalline holes and the intracrystalline microporous diffusion. The diffusion coefficient was calculated for a previous estimated membrane thickness. This last parameter was obtained from the modeling of steady state permeability at high temperature and for a non-adsorbing gas like helium.