This paper presents a fully integrated power management and sensing microsystem that harvests solar energy from a micro-power photovoltaic module for autonomous operation of a miniaturized hydrogen sensor. In order to measure H-2 concentration, conductance change of a miniaturized palladium nanowire sensor is measured and converted to a 13-bit digital value using a fully integrated sensor interface circuit. As these nanowires have temperature cross-sensitivity, temperature is also measured using an integrated temperature sensor for further calibration of the gas sensor. Measurement results are transmitted to the base station, using an external wireless data transceiver. A fully integrated solar energy harvester stores the harvested energy in a rechargeable NiMH microbattery. As the harvested solar energy varies considerably in different lighting conditions, the power consumption and performance of the sensor is reconfigured according to the harvested solar energy, to guarantee autonomous operation of the sensor. For this purpose, the proposed energy-efficient power management circuit dynamically reconfigures the operating frequency of digital circuits and the bias currents of analog circuits. The fully integrated power management and sensor interface circuits have been implemented in a 0.18 mu m CMOS process with a core area of 0.25 mm(2). This circuit operates with a low supply voltage in the 0.9-1.5 V range. When operating at its highest performance, the power management circuit features a low power consumption of less than 300 nW and the whole sensor consumes 14.1 mu A.