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A monolithic micro-gas-sensor system, designed and fabricated in a standard CMOS process, is described. The gas-sensor system incorporates an array of four microhotplate-based gas-sensing structures. The system utilizes a thin film of tin oxide (SnO2) as a sensing material. The interface circuitry on the chip has digital decoders to select each element of the sensing array and an operational amplifier to monitor the change in conductance of the film. The chip is post-processed to create microhotplates using bulk micro-machining techniques. Measurements are presented for various portions of the interface circuitry used for the gas-sensor system.