The sensing behaviour of three kinds of Bi-based superconductor film, differing in microstructure, has been studied. The films comprise Bi2Sr2CaCu2O8+x(BSCCO) with "open neck" contacts between grains, Bi2Sr2CUO6+x(BSCO) prepared by electronic beam evaporation and Bi2Sr2CaCU2O(8+x) with "Schottky-barrier-limited" conductance. The sensitivity and selectivity to NO and NO2 gases against CO and CO2 have been investigated. The flow rate dependence of the NOx sensitivity has been measured. The sensing behaviour of the third film to NO has been measured with both air and Ar as carrier gas. NO behaves as a reductant in air on the surfaces of Bi2Sr2CaCu2O8+x and Bi2Sr2CuO6+x. NO2 is neither reductant nor oxidant under the same conditions. A qualitative sensing mechanism for NOx on Bi2SrCaCu2O8+x and Bi2Sr2CuO6+x is proposed.