The responses of lead phthalocyanine thin films upon encountering of NO2 gas were studied both experimentally and theoretically. A surface adsorption/desorption and bulk diffusion mechanism was proposed to describe the response. Laplace transform was employed to solve the resulting diffusion equation and an analytical solution was obtained. The analytical results show that the steady state current depends on parameters such as film thickness, gas diffusivity, adsorption and desorption rate constants, etc. Diffusivity, adsorption and desorption rate constants can be extracted from the nonlinear least square fitting of experimental data with analytical results. The obtained gas diffusivity, which depends on film morphology, remains fairly constant as film thickness changes. From the results of this study, it was concluded that the adsorption of NO2 by lead phthalocyanine is controlled by the surface adsorption and desorption with negligible diffusion resistance.