Recognition and sensing of anions has been a subject of intensive current interes. Both chemosensor and biosensor are developed to ensure an optimal approaching of highly efficient communication between the recognition of the receptor and anion. Some analytical techniques such as fluorescence are becoming attractive tools for anion recognition. Mass Spectrometry is ideally suited for the rapid and sensitive analysis and separation of molecules with a wide variety of properties. ESI-MS is increasingly being applied to studies of weakly bound species, including non-covalent protein complexes and has been particularly useful in characterization of protein complexes, including determination of binding sites and stoichiometry, as well as ligand field geometry and composition. Most commonly molecular recognition is achieved through a favorable combination with various interactions in solution. In this article, ESI-MS was introduced here to invetigate quantitatively the ability of lysozyme to recognize anions.