This paper presents a fully integrated, ultra-low power microsystem that is used for hydrogen gas sensing in an autonomous wireless sensor node. The proposed circuit harvests solar energy from a micro-power photovoltaic module to measure temperature and hydrogen concentration and transmits the measured value using wireless data transmission. A rechargeable NiMH microbattery is used to store harvested energy. Photovoltaic module charges this microbattery, using a highly area-and power-efficient power management circuit. In order to measure hydrogen concentration, conductance change of a miniaturized palladium nanowire sensor is measured and converted to a digital signal with 12-bit resolution, using an area-efficient readout circuit. The proposed microsystem has been implemented in a 0.18 mu m CMOS process and occupies a core area of only 0.47mm(2). This circuit features a low current consumption of 200nA for power management circuit and an additional 1.1 mu A for sensor interface circuit. It operates with low power supply voltage in the 0.8V to 1.6V range.