The effect of NH3 on the deformation of polyester-coated galvanised steel at different relative humidities (RHs) was investigated using electrochemical impedance spectroscopy. Measurements were performed on specimens which had been tested in accelerated gaseous corrosion test. For this purpose the samples were subjected to NH3 gas for 16 and 24 days in atmospheric test cells with adjusted RH from 60 to 100%. Subsequently, the impedance response of the coated material was measured and evaluated. The results indicated that the coating performance varies with RH. At lower RH, remarkably, the coating remained effectively intact with the coating resistance varying inversely with RH. This work is relevant to the application of such organic-coated products adjacent to some plant flues were high levels of NH3 occur in association with high humidity. (C) 2003 Elsevier B.V. All rights reserved.