This work describes the preparation of mixed matrix membranes by casting from poly(1,4-phenylene ether-ether-sulfone) chloroform Solutions containing dispersed zeolitic imidazolate frameworks. Diffusive studies of CO2 in the pristine poly(1,4-phenylene ether-ether-sulfone) membrane and composite membranes were performed at 6 bar and 298 K, using pulsed field gradient NMR techniques. The evolution of the heterogeneity of the diffusion environments as seen by NMR was monitored ill terms of the diffusion time and a stretching parameter. The values of the self-diffusion coefficient increase with filler content, from 2.1 x 10(-8) cm(2) s(-1) for pristine membranes to 9.3 x 10(-8) cm(2) s(-1) for membranes with 30 wt % of Filler. Sorption and permeation experiments carried out at different Pressures Were used to determine the dual-mode model parameters that describe the transport processes. Apparent diffusion coefficients of CO2 in the membranes were obtained from the time lag method, from parameters of the dual-mode model, and directly from the derivatives of the steady flux, expressed in terms of concentration and pressure, with respect to pressure. In general, the Values Of the apparent diffusion coefficients obtained by the three methods for pristine membranes are in rather good agreement with the self-diffusion coefficient obtained by the NMR technique. In composite membranes, the values of the self-diffusion coefficients are nearly 2 times those obtained using permeation and sorption experiments. The discrepancies between the values Of the self-diffusion coefficients and the results obtained for the diffusion coefficient by other techniques are discussed. The filler contributes greatly to gas permeation by increasing the gas Solubility in the composite membranes.