The incorporation of zeolite-encaged iron-phthalocyanine partial oxidation catalysts into a dense hydrophobic polymer membrane results in a substantial improvement in catalyst performance. The diffusion through these zeolite-filled membranes is described using a two-dimensional model, and it is demonstrated that for a permeability ratio (polymer over catalyst) higher than 10(4), the diffusion through composite polymer-zeolite membranes can be described by a one-dimensional model. Such a one-dimensional mathematical model was developed and validated with the experimental data, obtained from the time lag measurements on zeolite-tilled poly(dimethylsiloxane) polymer membranes. Consequently diffusion through composite catalytic membranes can be predicted using the mass transfer coefficients of pure catalyst and polymer material, and a single tortuosity factor, only dependent on the catalyst loading.