Abstract Chemical modifications of B 4 C cap layers on sputtered Pd/B 4 C multilayer coatings for X-ray optical applications were investigated using X-ray reflectivity, photoemission electron spectroscopy, photoemission electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and infrared spectroscopy. The results indicate oxidation down to probing depths of about 10 nm and strong evidence for the formation of B 2 O 3 crystals at the sample surface, while B 4 C like compounds are absent.