In this work we present a micromachined twin sensor that can distinguish between carbon monoxide (CO) and O2 gas taking advantage of the high sensitivity of SnO2 to these gases. The SnO2 nanoparticles of both sensors are catalytically modified with different Pd loadings that act as active filters. In this way, one sensor response is turned to present a higher sensitivity to CO than to O2, whereas the other sensor is mainly turned for detecting O2 variations. The twin sensor has two membranes in the same die of micromachined silicon. Each membrane works independently from the other without any cross talk of temperature. The resistance data obtained from this twin sensor as a function of CO and O 2 concentrations is parametrized. Then, two functions are calculated for the quantification of CO/O2 gas mixtures. A comparison with commercial gas sensors is shown. © 2005 Elsevier B.V. All rights reserved.