Fluorescence　spectra　of　thymine-containing　oligonucleotides　labeled　with　donor/acceptor　in　the　presence　of　Hg2+　ions　were　investigated.　Formation　of　T-Hg2+-T　base　pairs　gives　rise　to　a　hairpin　structure　and　makes　both　termini　of　the　oligonucleotide　close　to　each　other.　For　fluorescein　or　tetramethylrhodamine　single-labeled　oligonucleotide,　fluorescence　quenching　was　observed　on　addition　of　Hg2+　ions.　For　fluorescein　and　tetramethylrhodamine　double-labeled　oligonucleotide,　the　apparent　FRET　efficiencies　decrease　unexpectedly　in　the　presence　of　Hg2+　ions.　The　unusual　fluorescence　quenching　in　the　presence　of　Hg2+　ions　was　ascribed　to　formation　of　T-Hg2+-T　base　pairs,　which　can　accept　and　mediate　the　electron　transfer　and　provide　an　additional　de-excitation　process　for　the　excited　state　of　fluorophores　via　photoinduced　electron　transfer.　©　2011　American　Chemical　Society.