Magnetically assisted chemical separation (MACS) was developed at Argonne National Laboratory as a compact process for reducing the volume of high-level aqueous waste streams that exist at many U.S. Department of Energy sites. In the MACS process, ferromagnetic particles coated with solvent extractants are used to selectively separate transuranic nuclides and heavy metals from aqueous wastes. The contaminant-loaded particles are recovered from the waste stream by using a magnet. For the recovery of transuranic species the extractant used is octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) dissolved in tri-n-butyl phosphate (TBP). To better understand the extraction chemistry of this solvent/particle system, europium was used to monitor the sorption capacity of the MACS particles for lanthanides and actinides. Europium concentrations varying from 10(-7)(M) under bar to 10(-1)(M) under bar were prepared with 3 (M) under bar NaNO3 in 0.1 (M) under bar HNO3. Neutron activation analysis was used to measure the concentration of europium. The sorption capacity was evaluated, along with the sorption isotherms to simulate multiple contact stages. The maximum absorption capacities obtained was 0.62 mmol/g. The adsorption models of Langmuir and Freundlich and the extended Langmuir model of Brunauer, Emmett, and Teller (BET) were fitted to the data. The best correlations were obtained from the Langmuir and BET models, a result that supports a monolayer adsorption mechanism. The Langmuir and BET models suggested a loading capacity of 0.33 mmol/g and 0.25 mmol/g, respectively. The Freundlich model results support favorable metal loading with a 1/n value of 0.3.