Purity control of the primary and secondary scdium systems at the Experimental Breeder Reactor II (EBR-II) is by essentially continuous cold trapping of small side streams of the total sodium inventories. The EBR-II cold traps are effective for the control of the major chemical impurities, i.e., oxygen and hydrogen (and also tritium). The trapping effectiveness is higher for hydrogen (NaH) than for oxygen (Na2O). The trap on the primary sodium system is more effective than the secondary cold trap because of higher sodium velocities and, probably, longer residence times in the crystallization zone of the primary trap. Radioisotopes such as 131I and 137Cs are not effectively trapped. New control methods may be required for these and other radioisotopes to allow the continued use of direct maintenance procedures for various plant systems.