An unprecedented redox-neutral S-nitrosation of thiol has been achieved at dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO • and thiol generates a dicopper (I,I) di-Snitrosothiol complex [Cu I Cu I (RSNO)2] 2+ or dicopper (I,I) mono-Snitrosothiol complex [Cu I Cu I (RSNO)] 2+ , which readily release RSNO in 88-94% yield. The S-nitrosation reaction proceeds through a mixedvalence [Cu II Cu III (-O)(-NO)] 2+ species, which deprotonates RS-H at the basic -O site and nitrosates the RS − at the -NO site. The [Cu II Cu III (-O)(-NO)] 2+ complex is also competent for O-nitrosation of MeOH, which is isoelectronic to thiol. In this case, a rare [Cu II Cu II (-NO)(OMe)] 2+ intermediate has been isolated and fully characterized, suggesting the S-nitrosation proceeds through the intermediary of analogous [Cu II Cu II (-NO)(SR)] 2+ species. The redox-and protonneutral S-nitrosation process reported here represents the first functional model of ceruloplasmin in mediating S-nitrosation of external thiols, adding further implications for biological copper sites in the interconversion of NO • /RSNO.