^13C-Nuclear magnetic resonance (CMR) and Raman spectra of ergothioneine and its mercury complex have been recorded as function of pH. The ¹3C-signals of ergothioneine is assigned by comparison with the chemical shifts of histidine, 2-mercaptoimidazole and betaine. Ergothioneine is present in thione form in an aqueous solution or a solid state. In basic solution, ergothioneine exists as thiolate ion upon the deprotonation of the imidazole ring, and the tautomeric form of its imidazole ring is predominantly the 1-H tautomer. The result of ¹3C-NMR and raman spectra support that ergothioneine coordinates to Hg (II) ion through the thiolate sulfur.