Selective control of enzyme activity is critical for elucidating the roles of specific proteins in signaling pathways. One potential means for developing truly target-specific inhibitors involves the use of protein engineering to sensitize a target enzyme to inhibition by a small molecule that does not inhibit homologous wild-type enzymes. Previously, it has been shown that protein tyrosine phosphatases (PTPs) can be sensitized to inhibition by a biarsenical probe, FlAsH-EDT2, which inhibits PTP activity by specifically binding to cysteine residues that have been introduced into catalytically important regions. In the present study, we developed an array of biarsenical probes, some newly synthesized and some previously reported, to investigate for the first time the structure-activity relationships for PTP inhibition by biarsenicals. Our data show that biarsenical probes which contain substitutions at the 2' and 7' positions are more effective than FlAsH-EDT2 at inhibiting sensitized PTPs. The increased potency of 2',7'-substituted probes was observed when PTPs were assayed with both para-nitrophenylphosphate and phosphopeptide PTP substrates and at multiple probe concentrations. The data further indicate that the enhanced inhibitory properties are the result of increased binding affinity between the 2',7'-substituted biarsenical probes and sensitized PTPs. In addition we provide previously unknown physicochemical and stability data for various biarsenical probes.