Metal-insulator-semiconductor devices are promising sensors for low concentrations of hydrogen and other molecules. The major challenge in design and use of these sensors is in optimization of selectivity for the desired analyte. Selectivity improvement is hampered by a poor understanding of the catalytic chemistry by which these devices function. In this presentation, we will discuss our efforts to design devices for improved selectivity (by use of alloys and surface-modifying monolayer films) and to understand mechanisms for selectivity through surface characterization. Possible applications for designed sensors will be discussed.