Beside the energy loss there is also some straggling towards lower energies upon reflection of a surface. Results are shown for particles impinging on a MgO surface. Except for primary oxygen molecules the FWHM in the energy spectrum of the reflected particles is proportional to the incident energy per atom and does not depend on the primary particle type. This is similar to the results obtained from a BaZrO surface [16,14]. Oxygen molecules show an increased straggling and an increased energy loss compared to hydrogen molecules. The dissociation mechanism might be different for these two molecules.