The interaction of an unsupported silver catalyst which had been pretreated by hydrogen at various temperatures with oxygen at 210°C has been studied using Temperature Programmed Reduction (TPR) over a temperature range up to 900°C. Hydrogen treatment at 500°C or above before the oxidation step causes the formation of extra species, thought to be OH groups in the sub-surface of the sample. A peak in the spectra attributable to oxygen strongly bound in the vicinity of surface defects is found to be dependent on the surface roughness and grain size of the silver sample used; hydrogen pretreatment causes the strongly bound oxygen in the vicinity of surface defects to be converted to sub-surface OH. It is also shown that the TPR measurements themselves influence the morphology of the sample and that these changes are comparable with structural changes which occur during the use of the catalysts for oxidative dehydrogenation of methanol. It is suggested that these structural changes are caused by the interaction of the sub-surface of the silver with both oxygen and hydrogen.