Nitric oxide-derived metabolites have been demonstrated to covalently modify cellular protein repertoire, thus affecting specific enzymatic functions. Among the various redox posttranslational modifications, protein nitration has been broadly recognized by immunological, spectroscopical, and chromatographic methods as a widespread reaction regulating essential phatophysiological processes. With the introduction of matrix-assisted laser desorption and electrospray as soft ionization methods for mass spectrometry of biomolecules, nitration has been investigated directly at the protein level, assigning polypeptide modification sites. Peptide mass fingerprinting and fragment fingerprinting upon collisional fragmentation approaches have been widely used to this purpose. This chapter describes how minimal levels of nitration present on a model protein, namely bovine serum albumin, generated in vitro by ONOO(-) treatment, were ascertained by integrated mass spectrometry approaches, identifying sites of modification. Critical considerations on the limits of each mass spectrometric ionization methods are also provided.