Abstract: A single MEMS thermal conductivity sensor based system capable of simultaneously measuring an air contained hydrogen and water vapor concentrations is described. The operation of the system is based on: hydrogen and water vapor have different temperature coefficients of their thermal conductivities and the sensor can be modulated by temperature so as to calculate their concentrations by their contributions to the total thermal conductivity of the air contained hydrogen and water vapor at different modulation temperatures of the sensor. The sensor comprises a one-dimensional gas heat conduction chamber recessed in a silicon substrate and a thermopile on the top of the chamber is used to measure the temperature difference between the heat source on the top and heat sink on the bottom of the chamber which is produced by the heat conduction of the air contained hydrogen and water vapor in the chamber.