In order to improve the weatherability of coatings, changes in chemical structure by weathering test are important. Research of bulk and surface structural changes in the same coating is indispensable to understand weathering degradation mechanism, because the bulk and surface structural changes are different generally. Therefore, in this study, we carried out reactive pyrolysis GC/MS and TOF-SIMS measurements to evaluate the bulk and surface structural changes, about alkyd-melamine coatings after weathering tests using Super Xenon Weather Meter SX75. The bulk and surface structural changes were very different. The structural change of melamine was not noticeable in the bulk, however the melamine crosslinks were broken in the surface. Furthermore, the palmitic acid structure was somewhat stable in the bulk, but significantly degraded in the surface. The surface is most radiated by UV and contact with oxygen and water which accelerate UV degradation. Hence, the degradation in the surface is more prominent than in the bulk.