Some gold(III)–dithiocarbamato derivatives of either single amino acids or oligopeptides have shown promise as potential anticancer agents, but their capability to interact with biologically relevant macromolecules is still poorly understood. We investigated the affinity of the representative complex [AuIIIBr2(dtc-Sar-OCH3)] (dtc: dithiocarbamate; Sar: sarcosine (N-methylglycine)) with selected model molecules for histidine-, methionine-, and cysteine-rich proteins (that is, 1-methylimidazole, dimethylsulfide, and N-acetyl-L-cysteine, respectively). In particular, detailed mono- and multinuclear NMR studies, in combination with multiple 13C/15N enrichments, allowed interactions to be followed over time and indicated somewhat unexpected reaction pathways. Whereas dimethylsulfide proved to be unreactive, a sudden multistep redox reaction occurred in the presence of the other potential sulfur donor, N-acetyl-L-cysteine (confirmed if glutathione was used instead). On the other hand, 1-methylimidazole underwent an unprecedented acid–base reaction with the gold(III) complex, rather than the expected coordination to the metal center by replacing, for instance, a bromide. Our results are discussed herein and compared with the data available in the literature on related complexes; our findings confirm that the peculiar reactivity of gold(III)–dithiocarbamato complexes can lead to novel reaction pathways and, therefore, to new cytotoxic mechanisms in cancer cells.