The svnthesis of epitaxial thin films of HgBa2CaCu2O6 via a Hg-T1 cation exchange is reported (see figure). The mechanism of the newly developed, reversible Hg-T1 cation exchange is explored. The simple mechanism of the cation exchange may provide a generic "atomic surgery", and the process may be useful for the synthesis and epitaxy of elusive volatile material, and used to search for new volatile material from existing precursor materials.