Poly dimethyl siloxane (PDMS) membranes show potential for the separation of carbon dioxide from hydrogen in syngas applications due to their strong affinity for CO2. The Flory-Rehner theory of mixing enables the sorption of gases within such a rubbery membrane to be modeled, as long as the Flory-Huggins interaction parameters are known. In this work, mixed gas permeation experiments are used to determine interaction parameters for a range of relevant gas pairs. For the N-2-H-2 system, the mixed gas performance is comparable to pure gas measurements and indicates that H-2 does not interfere with the permeation of N-2 through the membrane. In contrast, for the CO2-H-2 system, the CO2 permeability falls with increasing H-2 partial pressure, attributed to competitive sorption. The effect of a typical hydrocarbon impurity, toluene, upon CO2 and N-2 permeability through PDMS was also measured. The addition of this aromatic hydrocarbon caused significant polymer swelling, which required solubility modeling with the Flory-Rehner model. A concentration dependent diffusivity was also required to fit the permeation data. The parameters determined from the laboratory experiments were used to model pilot plant results for a PDMS membrane separating CO2 from syngas, as part of the CO2CRC Mulgrave Project. It was found that the model could effectively predict experimental CO2, N-2 and H-2 permeabilities. (c) 2013 Elsevier B.V. All rights reserved.