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A novel way of operating metal-oxide gas sensors is demonstrated that extends the gas-analyzing facilities of conventional thin-film gas sensors. In our approach, thin-film metal-oxide gas sensors on micro-machined heater substrates are embedded into tiny silicon micro-chambers to form micro-reactor devices. Analyzing samples of polluted air, such micro-reactors can be operated either in constant-flow or no-flow modes. Whereas in the first mode, essentially normal sensor behavior is observed, gas depletion reactions are observed in the latter. Such depletion reactions are shown to provide, in a straightforward way, analytical information about gas mixtures which is difficult to obtain under normal sensor operating conditions. As an application example, we demonstrate how a micro-reactor device can be used to analyze samples of polluted air for their O/sub 3/ and NO/sub 2/ contents.