We describe a new highly selective supported liquid membrane (SLM) system made of a polypropylene flat sheet with a thymine derivative in kerosene as the carrier, for the selective transport of mercury ion. When the carrier existed in the membrane, only mercury ions were selectively transported. This SLM selectively separated mercury ions from a mixed solution of mercury and heavy metal ions. The hydrogen ion concentration in the receiving phase is a dominant factor in the mercury ion transport across the SLM.