In this work we report the effect on the NO2 gas sensing properties of initially doping CuPc thin films with oxygen (in air) and NO2 for room temperature operation. The pre-treatment with NO2 is shown to improve the gas sensing properties by providing both an increase in the magnitude of the conductivity change for a given NO2 concentration and a significant improvement in the recovery time. Data presented suggest that a simple time derivative of the change in current may provide a measure of concentration for real time gas sensing applications.