In recent years there has been a remarkable growth in coatings technology, yet polymer‐coated metals still corrode when they are exposed to severe environments. If the effectiveness of polymer coatings is to be increased, it is essential to understand the microstructure of polymer coating film and the changes that occur to the film upon environmental exposure, and relate the changes to the protective performance of coatings. The degradation of a polyester immersed in alkali solution has been investigated using a number of analytical techniques including atomic force microscopy (AFM), liquid chromatography/mass spectrometry (LC/MS), and Fourier transform infrared spectroscopy (FTIR). AFM was used to characterize the heterogeneous phase in the unexposed films and films exposed to alkali solution. Film roughness was found to increase with aging of the film in alkali medium. Total organic carbon analysis of the leached aqueous medium showed the presence of organic compounds, suggesting a chemical degradation of the film in alkali medium. FTIR analysis of the leached medium showed evidence for the formation of carboxylate species upon degradation of polyester film in alkali solution, while LC/MS analysis of the leached medium confirmed the presence of isophthalic acid and sodium isophthalate. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2454–2463, 2000