A novel type of thin-film catalyst has been developed that combines high activity with selective transport across a micron-scale membrane film. Nanoporous carbon was used as a support for 12-tungstophosphoric acid with composite films synthesized by two distinct approaches: in-situ polymerization and adsorption from solution. Ion sputtering and XPS were used in combination to characterize the high dispersion of the catalyst within the film in the former case and a gradient in catalyst concentration in the latter. Both methods produce intact Keggin structures as demonstrated by FTIR spectroscopy. Gas permeation experiments indicate a preservation of separation capacity with O-2/N-2 permeability ratios as high as 6.5. The catalytic films were benchmarked using the decomposition of methyl tert-butyl ether at 55 degrees C and demonstrate remarkable increases in both conversion and simultaneous separation of decomposition products. Integrated conversions in a semi-batch reactor configuration far exceed the thermodynamic limit of 67.4% at 55 degrees C and 0.25 atm initial MTBE pressure to conversions approaching 99.9%. At the same time, methanol and isobutylene are separated on either side of the membrane with methanol selectivities as high as 99.8% on the permeate side. These results indicate that this novel catalyst design and geometry have significant potential to increase reactor efficiency and simultaneously to reduce or eliminate subsequent separation processes in selected applications.