The elucidation of mechanisms by which cysteine (Cys) redox reactions influence metal binding to zinc finger domains is important for understanding the structure and function of zinc fingers. The present studies utilize electrospray ionization mass spectrometry (ESI-MS) to analyze Cys redox reactions and their influence on metal ion binding to a synthetic polypeptide similar in motif to the third zinc finger of the RNA polymerase II transcription factor, Sp1 (Sp1-3). The differential specificity of metal binding events to this zinc finger domain is demonstrated over a range of redox-altering dithiothreitol, hydrogen peroxide, and hydrogen ion concentrations. By analyzing this Cys2His2 zinc finger domain at single Da resolution with ESI-MS, shifts in the natural isotope cluster demonstrate that a Cys thiol and thiolate can contribute to Zn2+ and other metal ion coordination. These experiments provide insight into the basic redox chemistry and metal binding mechanisms of Cys2His2 zinc finger domains.