Porous silicon (PS) conductometric gas sensors are used to create a sensitivity matrix for the room temperature detection of NOx (NO, NO2). "P-type" nanopore coated microporous silicon is treated with tin, nickel, copper, and gold, electrolessly deposited onto the PS surface to form SnOx, NiO, CuxO, and AuxO nanostructured centers as confirmed by XPS measurements. The relative sensitivities of these modified PS gas sensor surface sites have been measured under 1-5 ppm NO exposure. An improved sensitivity of up to 10 times that of untreated PS is observed for a 1 ppm exposure. The choice of deposits is based on the hard to soft acid character of the nanostructured metal oxide islands that are fractionally deposited on the semiconductor interface and their effect on the physisorption of NO, a weak base, dictated by an inverse pattern (IHSAB) to the hard-soft acid base concept. NO, a free radical, can interact with oxygen sites on the modified PS sensor interfaces, to effect a transient NO2 signal unique to PS-based NO sensors, which is not observed as other basic analytes including NH3, PH3, H2S, SO2, and CO interact with "p-type" PS. A comparison is made between the current PS sensor systems which operate at room temperature and electrochemical and traditional metal oxide sensors. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3583368] All rights reserved.