Copper windings in the transformers used on national distribution networks are often insulated with layers of paper. In large current transformers the whole paper‐insulated assembly is immersed in an insulating mineral oil within a de‐aerated tank. Such transformers have a normal life span in excess of 30 years. However, from time to time, insulation problems arise and this is then associated with significant darkening of the paper. Analysis of the damaged paper by XPS shows that the darkening arises from the deposition, within the paper winding, of copper compounds, usually Cu2O or CuO. Laboratory studies in support of work designed to eliminate the problem have shown that copper diffusion occurs in mineral oil if it contains a trace of oxygen. In this paper we will show that the transport of copper through up to 20 layers of paper can be measured by XPS and that the concentration from one paper winding to the next declines in accord with Fick's law for non‐steady‐state diffusion. Measurements of surface concentrations by XPS have allowed determination of the diffusion coefficients and activation energy for the transport process and thus provide a basis for interpretation of the diffusion profiles found in the transformer in terms of time and temperature of operation. Copyright © 2002 John Wiley & Sons, Ltd.