PTP Inhibitor III is an α-haloacetophenone derivative that acts as a photoreversible covalent inhibitor of protein tyrosine phosphatases (PTPs). It binds the catalytic domain of SHP-1 (Ki = 184 μM) and covalently reacts with free thiols, a reaction that is reversible with irradiation (350 nm). PTP Inhibitor III is cell permeable and inhibits a broad range of PTPs.