The dehydration-dehydrogenation of 2-hexanoI in the presence of a catalyst obtained from magnesium hydrogen phosphate was studied. The catalyst activity was found to be related to the nature of surface OH sites. Examination of the catalyst by the XRD, DRIFT, H-1 and P-31 MAS NMR, and UV-visible absorption techniques allowed the nature of the catalytically active sites to be established and the catalytic activity results to be explained.