Surface products of oil-impregnated insulation paper during the damage process caused by partial discharge (PD), as well as gas within the cavity, were studied. An optical microscope and a scanning electron microscope (SEM) were used to investigate surface morphology, while an infrared spectroscopy (IR) and an X-ray photoelectron spectroscopy (XPS) were used to study surface products and their components. The volume variation in cavity gas was also analyzed. Furthermore, gas constituents and their relevant contents were studied using a gas chromatography-mass spectrometer (GC-MS). The study results reveal the following: during the PD damage process, the total gas volume and the content of electronegative gasses alternately decline and increase, while discharge types alternate between pulse type and pseudo-glow type (or glow type); "surface droplets" and "crystalline solids" appear on the insulation surface one after another; surface droplets mainly consist of (C=O)-group-containing compounds, whereas crystalline solids are mainly carboxylic acids, with carboxyl groups also found in cellulose chains; and the discharge type related to the oxidization of decomposition products is the main factor that determines the state (liquid or solid) of the surface products. (C) 2011 Elsevier B.V. All rights reserved.