Functionalization of multiwall carbon nanotubes (MWCNTs) allows the outer wall of the nanotubes to be turned into chemically selective surfaces. As in the electric transport along the nanotube axis the outer wall plays a dominant role, the modulation of this transport lends itself to direct application in gas sensing. In our work the modification of electrical conduction was used as a detection principle in random networks (mats) of MWCNT. In comparison with conventional adsorption type (e.g. Taguchi) sensors these MWCNT networks exhibited very fast response times (< 10s) to all gases and vapors applied, and showed acceptable long-term stability.