Detection and characterisation of structural modifications of a hindered amine light stabiliser (HALS) directly from a polyester-based coil coating have been achieved by desorption electrospray ionisation mass spectrometry (DESI-MS) for the first time. In situ detection is made possible by exposing the coating to an acetone vapour atmosphere prior to analysis. This is a gentle and non-destructive treatment that allows diffusion of analyte to the surface without promoting lateral migration. Using this approach a major structural modification of the HALS TINUVIN®123 (bis(1-octyloxy-2,2,6,6-tetramethyl-4- piperidyl) sebacate) was discovered where one N-ether piperidine moiety (N-OC8H17) is converted to a secondary piperidine (N-H). With the use of 2-dimensional DESI-MS imaging the modification was observed to arise during high curing temperatures (ca. 260°C) and under simulated physiological conditions (80°C, full solar spectrum). It is proposed that the secondary piperidine derivative is a result of a highly reactive aminyl radical intermediate produced by N-O homolytic bond cleavage. The nature of the bond cleavage is also suggested by ESR spin-trapping experiments employing α-phenyl-N-tert-butyl nitrone (PBN) in toluene at 80°C. The presence of a secondary piperidine derivative in situ and the implication of N-OR competing with NO-R bond cleavage suggest an alternative pathway for generation of the nitroxyl radical - an essential requirement in anti-oxidant activity that has not previously been described for the N-ether sub-class of HALS. © 2011 The Royal Society of Chemistry.