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A current problem resistive gas sensors have to deal with is their power consumption. Resistive gas sensors naturally require an operating temperature of several hundred degrees centigrade. Especially for mobile applications a novel sensor generation is needed whose power consumption is less than 100mW. This paper describes the processing of an advanced gas sensor substrate on a membrane structure which fulfils these requirements. The function of the membrane is to carry sensor structures and to reduce heat dissipation. In contrast to conventionally used backside chemical wet etching (with its known problems) a thick structured porous silicon layer (100 microns) as a sacrificial layer at the frontside of the chip is used, To reduce the surface topography porous silicon is processed with the help of implanted masks. Following the processing of this layer gas sensor structures are realised on top of the porous silicon. In a final chemical dry etch step the membrane is structured and simultaneously the porous silicon is uncovered from the front and can be removed afterwards. The result is a free-standing thermally decoupled gas sensor membrane with a power consumption less than 50mW.