Columnar porous silicon (CPS) substrate SnO2 gas-sensor devices with high sensitivities and low working temperatures (input power 500-800 mW in TO-99 casing) are presented. The paper compares a conventional substrate (SnO, on SiO,/silicon substrate) with two porous substrate devices, one with pores of 60-300 nm ('porous'), and the other of 400-2000 nm ('mesoporous'). The CPS substrates were prepared on <100> p-type 3 inch silicon wafers through boron diffusion and anodisation. The SnO2 was prepared through sol-gel deposition from Tin II Ethylhexanoate precursors and was deposited through spin-coating. The W/Au inter-fingered contact structure was deposited over the SnO2yy layers for the 'porous' and 'mesoporous' devices. The SnO2 layers were vitrified layer-by-layer at 400 degreesC and at the end annealed at 500 degreesC. The results indicate a mid-range working temperature range (250 degreesC) for the 'porous' substrate device, with good response times and high sensitivity, relative to the SnO2/silicon substrate device. The 'mesoporous' substrate device has yet higher sensitivity, however, it is considerably slower (typically 320 s OFF-times for O-2 in synthetic air).