Positive ion electrospray mass spectrometry has been used to investigate the interaction of cis-[Pd(en)(H2O)(2)](2+), cis-[Pd(dtco-3-OH)(H2O)(2)](2+) (dtco-3-OH = dithiacyclooctan-3-ol), and trans-[Pd(py)(2)(H2O)(2)](2+) with glutathione (GSH), s-methylglutathione (GSMe), acetyl-s-methyl-cysteinylhistidylglycine (AcCysMe-His-GlyH), oxytocin, acetyl-methionine (AcMetH), and acetyl-methionylglycine (AcMet-GlyH). Reactions of cis-[Pd(en)(H2O)(2)](2+) or cis-[Pd(dtco-3-OH)(H2O)(2)](2+) with cysteine-containing peptides, GSH, GSMe, AcCysMe-His-GlyH, and oxytocin, form the following complexes: (1) a binuclear thiolato anion-bridged complex, cis-[Pd(dtco-3-OH)(mu-s-GS)](2)(2+); (2) mononuclear six-membered chelates involving a thioether group and the deprotonated amide nitrogen of glycine, cis-[Pd(dtco-3-OH)(S,N-GSMe)](+) and cis-[Pd(en)(S,N-GSMe)](+); (3) mononuclear chelates including a thioether and a deprotonated amide nitrogen, the carbonyl oxygen of histidine and a nitrogen of imidazole, cis-[Pd(en)(S,N-AcCysMe-His-GlyH)](+), [Pd(S,N,N,O-AcCysMe-His-GlyH)](+) and [Pd(S,N,N-AcCysMe-His-GlyH)(CH3OH)](+); (4) mononuclear chelates containing disulfide and N-terminal amino groups, cis-[Pd(en)(N,S-oxytocin)](2+) and cis-[Pd(dtco-3-OH)(S,N-oxytocin)](2+). From mixing of cis-[Pd(en)(H2O)(2)](2+) or cis-[Pd(dtco-3-OH)(H2O)(2)](2+) or trans-[Pd(py)(2)(H2O)(2)](2+) with AcMetH or AcMet-GlyH, in a 1:1 mole ratio, the following complexes are observed: (1) mononuclear six-membered chelates involving the thioether and the deprotonated amide nitrogen of methionine, cis-[Pd(dtco-3-OH)(S,N-AcMetH)](+), cis-[Pd(en)(S,N-AcMetH)](+), cis-[Pd(dtco-3-OH)(S,N-AcMet-GlyH)](+), cis-[Pd(en)(S,N-AcMet-GlyH)](+), and cis-[Pd(S,N-AcMetH)(CH3OH)(2)](+); (2) mononuclear chelates including the thioether, deprotonated amide nitrogen, and the carbonyl oxygen of methionine, [Pd(py)(S,N,O-AcMetH)](+) and [Pd(py)(S,N,O-AcMet-GlyH)](+). The finding of an interaction of Pd(II) with the carbonyl oxygen of an amide in [Pd(S,N,N,O-AcCysMe-His-GlyH)](+) and [Pd(py)(S,N,O-AcMet-GlyH)](+) is of interest in understanding the cleavage of the His-Gly bond in the former and the Met-Gly bond in the later. The results presented above indicate that electrospray mass spectrometry (ESMS) is a powerful tool for the analysis of reactions between palladium(II) complexes and sulfur-containing peptides. Both precise molecular masses and structural information of reaction products can be obtained. It is helpful to understand the coordination behavior of palladium(II) complexes toward sulfur-containing peptides and proteins.