[en] We have undertaken studies to identify the stable beam backgrounds that cause ‘contamination’ of the ECR ion source charge-bred radioactive beams from CARIBU (Californium Rare Ion Breeder Upgrade). Charge breeding of the radioactive species is necessary step prior to the acceleration of the beams in the ATLAS linac. Ion species whose (m/q) ratio is within 0.2% of the desired radioactive species (m/q) ratio cannot be discriminated against by the present accelerator configuration. Although there are many (m/q) instances in which an intense stable beam is sufficiently degenerate with the species of interest that no discrimination is possible, we also find many cases in which the stable beam background is in an acceptable range of intensity and can be dealt with by the experiment detection system. Examples of such situations will be presented as well as some discussion of ideas for improving the discrimination against stable beam backgrounds. This work is supported by the U.S. Department of Energy, Office of Nuclear Physics, under contract No. DE-AC02-06CH11357.