The fugacity coefficients of carbon dioxide in carbon dioxide/isobutane mixtures were measured using a microporous hollow fiber silica membrane, coupled with a high accuracy pure component equation of state for carbon dioxide. The membrane is a size discriminator, allowing smaller carbon dioxide (kinetic diameter, sigma = 0.33 nm) molecules to permeate, while blocking the flow of the larger isobutane (sigma = 0.5 nm) molecules. Experiments were run at two temperatures and four different carbon dioxide mole fractions. Using measured values of the temperature, permeate and mixture pressures, and carbon dioxide mole fraction, fugacity coefficient comparisons were made using an extended corresponding states model using the van der Waals one-fluid mixing rules and with the Peng-Robinson equation of state. (C) 1998 Elsevier Science B.V. All rights reserved.