The present study has been concerned with the response of gas sensors based on sublimed films of copper phthalocyanine (CuPc) and iron phthalocyanine (FePc) to nitrogen dioxide (NO2). An alternative strategy of deriving information on the concentration of NO2 from data of the response rate within the initial few minutes was accomplished, rather than from electrical signals obtained under conditions of saturation. The response to NO2 for the CuPc sensor was found to obey the Elovich equation, d theta/dt = aexp(-b theta), where d theta/dr represents the rate of change of the surface coverage theta, and a and b are constants. A detailed investigation of the dependence of the constants in the equation on the NO2 concentration revealed that the constant a was directly proportional to the concentration in the range 0.8-12.8 ppm. It was therefore possible to derive the NO2 concentration after a period of only 1 min exposure. However, the response of the FePc sensors did not follow the Elovich equation in the first 2 min exposure, and therefore, longer time periods were required to determine concentrations than for the case of CuPc. It is thought that the nature of the central metal ion in the phthalocyanine molecule might be responsible for this difference.