The results of several years of work devoted to developing an understanding of the process by which the resistive contribution of transition metal impurities, primarily iron, in copper is removed by internal oxidation are presented. The majority of the investigations were made on a CuFe alloy series of precisely determined composition from 1 to 100 atomic parts per million (at PPM) Fe. Electrical resistance and magnetic susceptibility measurements at room temperature and at 4K on both unoxidised and oxidised samples are reported. These measurements, supported by scanning electron microscopy (SEM) and Curie temperature determinations, suggest a two-step internal oxidation process at even the lowest impurity levels.