There is a need for the selective derivatization and enrichment of posttranslational protein modifications from tissue samples. This chapter describes a method for the selective derivatization of 3-nitrotyrosine (after reduction to 3-amino-tyrosine) and 3,4-dihydroxyphenylalanine with benzylamine derivatives to yield 6-amino- and 6-benzylamine-substituted benzoxazoles, which display characteristic fluorescence properties. The methodology can be expanded to other substituted benzylamines, which carry functional groups for affinity enrichment.