There is great interest in fluorogenic compounds that tag biomolecules within cells. Biarsenicals are fluorogenic compounds that become fluorescent upon binding four proximal Cys thiols, a tetracysteine (Cys(4)) motif. This work details interactions between the biarsenical AsCy3 and Cys(4) peptides. Maximal affinity was observed when two Cys-Cys pairs were separated by at least 8 amino acids; the highest affinity ligand bound in the nanomolar concentration range (K(app) = 43 nM) and with a significant (3.2-fold) fluorescence enhancement.