Arsenic is highly toxic and a significant threat to human health, but certain bacteria have developed defense mechanisms initiated by As^III binding to As^III-sensing proteins of the ArsR family. The transcriptional regulator AfArsR responds to As^III and Sb^III by coordinating the metalloids with three cysteines, located in a short sequence of the same monomer chain. Here, we characterize the binding of As^III and Hg^II to a model peptide encompassing this fragment of the protein via solution equilibrium and spectroscopic/spectrometric techniques (pH potentiometry, UV, CD, NMR, PAC, EXAFS, and ESI-MS) combined with DFT calculations and MD simulations. Coordination of As^III changes the peptide structure from a random-coil to a well-defined structure of the complex. A trigonal pyramidal AsS₃ binding site is formed with almost exactly the same structure as observed in the crystal structure of the native protein, implying that the peptide possesses all of the features required to mimic the As^III recognition and response selectivity of AfArsR. Contrary to this, binding of Hg^II to the peptide does not lead to a well-defined structure of the peptide, and the atoms near the metal binding site are displaced and reoriented in the Hg^II model. Our model study suggests that structural organization of the metal site by the inducer ion is a key element in the mechanism of the metalloid-selective recognition of this protein.