We have succeeded in sensing rapidly (0.1-0.5 s response), relatively low concentrations of toxic gases, and organic vapors using electric devices integrating films of aligned CNx nanotubes. We demonstrate that CNx nanotubes could be more efficient for monitoring toxic species, when compared to pure carbon nanotubes. This is due to the presence of highly reactive pyridine-like sites on the tube surface. Using ab initio calculations, we confirmed that pyridine-type regions on the tube surface bind strongly to ammonia, acetone and OH groups, thus altering their density of states. CNx nanotubes could be used as fast responsive and reusable sensors. (C) 2004 Elsevier B.V. All rights reserved.