In previous studies, the response to NO2 of a number of substituted azobenzenes was characterised for sensitivity and kinetics over a range of temperatures. These studies indicated that the optimal sensor of this sort must be based on a chromophore with a binding energy for the target gas of approximately 70 kJ mol(-1). Here, we present data for the best chromophore which has resulted, dispersed in an addition-cure polysiloxane matrix. We characterise the activation energy for the binding of NO2 to the azobenzene sites and report measurements of the response time of thin films from which the diffusion constant of NO2 in the matrix is deduced. We report on the behaviour of the films when operated in the cycled mode. We present quartz-crystal microbalance data allowing absolute calibration of the fractional site occupancy in a related system.