Provided are compounds capable of covalently binding to a protein tyrosine phosphatase (PTP). The compounds comprise Formula A: ##STR00001## Also provided are compositions comprising one of the above compounds covalently bound to a member of the PTP superfamily, methods of labeling a PTP using the compounds, methods of isolating a PTP from a mixture of proteins using the compounds, methods of evaluating whether a substance is an inhibitor of a PTP using the compounds, methods of evaluating the specificity of an inhibitor of a PTP using the compounds, methods of identifying a PTP involved in a disease in a mammal using the compounds, and methods of diagnosing a disease in a mammal using the compounds.