Sodium Orthovanadate (Vanadate, Na3VO4) is a commonly used general inhibitor for protein phosphotyrosyl phosphatases (PTPs). It is a competitive inhibitor. The inhibition by Vanadate is completely reversible upon the addition of EDTA or by dilution. Vanadate is activated for maximal inhibition of PTPs following the procedure described by J.A.Gordon. The PTP activities are conveniently separable from the protein phosphoseryl and phosphothreonyl phosphatase (PSP) activities inhibited by Fluoride and EDTA. Routinely Vanadate is used to preserve the protein tyrosyl phosphorylation state in cells, cell lysates, and protein tyrosine kinase assays.