The selective transport of Am across a supported liquid membrane (SLM) has been investigated by using bis (2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301) as a mobile carrier. This extractant containing soft donor atoms exhibits strong affinity for actinoids, giving a large separation factor between trivalent Am and Eu. Separation of Am from Eu was achieved by an SLM containing highly purified Cyanex 301. Americium was preferentially transported across the SLM and concentrated in the product solution, while most of Eu remained in the feed solution.