When multi-channel EP recordings are used for source localization, the electrode positions have to be determined with respect to a common reference frame. In this paper a method is described for determining the electrode positions on the head. Since it is difficult to fix electrodes accurately at a priori chosen positions, it is better to measure these positions afterwards. For this we have developed a practical method. The method also finds the best fitting sphere for the electrode position, which is useful when a multi-sphere volume conductor is used in the inverse algorithm.