The tetrameric anionic species {Na-3[Ag(Htma)](4)}(-) (m/z = 1092.8; H(3)tma = thiomalic acid) is detected by negative-ion electrospray ionization (ESI) mass spectrometry as a stable constituent unit derived from an aqueous solution of the oligomeric thiomalato-silver(I) complex, recently isolated and characterized as {Na[As(Htma)] . 0.5H(2)O}(n) (n = 24-34) 1, which displays antimicrobial activity against selected bacteria, yeast and moulds.