Separation of rare or expensive isotopes, whether radioactive or stable, requires a high efficiency of the separation process. Electromagnetic isotope separation is the most widely-used of all separation processes. Its separation efficiency depends mainly upon the ionization efficiency of its ion source. An Electron Cyclotron Resonance (ECR) ion source has been developed for the separation of both stable and radioactive isotopes on account of its high ionization efficiencies. The maximum ionization efficiencies achieved with the ECR ion source for singly charged ions for a range of materials were: radioiodine (8%), carbon (10%), nitrogen (26%), oxygen (53%), neon (31%) and xenon (83%). A computer code ECREFF, based on the atomic processes which occur in the plasma, is described for the calculation of the ionization efficiency of an ECR ion source. The experimental and calculated data are compared.