The pristine state and soft X-ray induced decomposition of two aromatic amino acids, viz. phenylalanine and tyrosine, have been studied by means of XPS and NEXAFS. The spectroscopic data on radiation induced decomposition have been supplemented by a mass spectral analysis of the desorbed species. Despite very similar chemical structures, the two amino acids show a dramatically different behavior toward ionizing radiation: phenylalanine degrades very quickly whereas tyrosine shows a prominent stability against radiation damage. Reasons for this difference are discussed in relation to radical-mediated reactions responsible for the decomposition.