Abstract: NO.sub.2 and other electron acceptor gases can be detected at 5 volume parts per million by tetrathiafulvalene TTF. The TTF may be incorporated in a polystyrene film, which changes its infra-red spectrum and conductivity upon exposure to such gases.

Abstract: A gas sensor element comprising a film of a semiconducting compound of formula I supported by a non-conductive substrate, the film being disposed between conductors for supplying a current to the film and withdrawing a current therefrom. ##STR1## in which: M represents 2H, Mn, Fe, Co, Ni, Cu, Hg, Cd, Pb, Pt or Pd;R.sub.1 represents H, CH.sub.3, Cl, Br, I, CN, CO.sub.2 H, COOC.sub.n H.sub.2n+1, CONH.sub.2 or NO.sub.2 ;R.sub.2 represents H or CH.sub.3 ; andR.sub.3.sup.a and R.sub.3.sup.b, which may be identical or different represent H, NO.sub.2, Cl, CO.sub.2 H or CH.sub.3 ;provided that when M represents cobalt at least one of R.sub.1, R.sub.2, R.sub.3.sup.a and R.sub.3.sup.b is other than hydrogen.