A very important mechanism of organic coating deterioration is the cathodic delamination. The basic knowledge about the diffusion mechanism of ions, oxygen and water through the coatings and through the defects in the coatings is not yet sufficient to improve the performance of organic coatings by developing more resistant coatings, and more research is necessary. In this study a particular electrochemical cell has been developed in order to keep separated the solution in contact with an artificial defect (inner solution) and the solution in contact with the rest of the intact organic coating (outer solution). In this way it is possible to polarize the sample in the cathodic region and to control the environment (ions and their concentration, oxygen, water activity) in the defect and around the defect. With this cell samples of polyester coated phosphatised steel were studied. The cathodic delamination was measured, in different solutions, by measuring the distance of delamination from the artificial defect after different times of exposure to cathodic polarisation. The experimental results are discussed in terms of two parameters: the induction time (time necessary to notice the beginning of delamination) and the delamination rate (mm(2) of delamination per hour). On comparing the experimental data it was possible to obtain new and interesting information on the delamination mechanism and to determine also the rate controlling step of the phenomenon for our materials, which is found to be the cation diffusion through both the coating and the defect. The proposed mechanisms were confirmed also by SEM observation and surface analysis.