The biarsenical-tetracysteine motif is a useful tag for genetic labeling of proteins with small molecules in living cells. The present study concerns the structure of a 12 amino acid peptide FLNCCPGCCMEP bound to the fluorophore ReAsH based on resorufin. H-1 NMR spectroscopy was used to determine the solution structure of the complex formed between the peptide and the ReAsH moiety. Structure calculations based on the NMR results showed that the backbone structure of the peptide is fairly well defined, with a hairpinlike turn, similar to a type-II B-turn, formed by the central CPGC segment. The most stable complex was formed when As2 was bonded to C4 and C5 and As1 to C8 and C9. Two clear NOESY cross-peaks between the Phe1 side chain and ReAsH confirmed the close positioning of the phenyl ring of Phe1 and ReAsH. Phe1 was found to have an edge-face geometry relative to ReAsH. The close interaction between Phe1 and ReAsH may be highly significant for the fluorescence properties of the ReAsH complex.