The amino acid DL-lysine has been investigated using the surface-sensitive probe XPS:(x-ray photoelectron spectroscopy). The XPS data confirm that solid-state lysine exists in the zwitterionic form. However, during the course of x;ray irradiation in the experiment, the molecule was observed to decompose. A decomposition mechanism consistent with;the data has been proposed which involves transformation of the zwitterion to a free base, followed by decarboxylation, to produce 1,5-diaminopentane and CO2 gas. It would appear that caution should be exerted when studying amino acids using XPS if long x-ray irradiation times are employed.