Magnesium orthophosphates have lately been examined as catalysts for various organic processes. In this work, we studied many compounds of this type synthesized under widely variable conditions, particularly as regards the magnesium and phosphorus sources, precipitant, and gel treatment used. The structure, composition and surface properties of each solid were found to depend on the synthetic procedure used. Thus, freshly made gels consisted largely of MgHPO4 . 3H(2)O, Mg-3(PO4)(2) . 8H(2)O, or Mg-3(PO4)(2) . 22H(2)O. The latter two can be interconverted by using various treatments. Calcination of the most extensively hydrated gel produces catalysts of increased specific surface area, acidity, and basicity. Thermal treatment yields phases such as Mg2P2O7, Mg-3(PO4)(2), and NaMg4(PO4)(3). The activity and selectivity exhibited by these solids in the conversion of gaseous 2-hexanol vary enormously with the particular synthetic procedure, In addition to surface properties, the structure and composition of the catalysts-particularly their sodium content-appear to play essential roles in their catalytic behavior, which is seemingly determined by their previously suggested dehydrogenating ability (with formation of sodium-magnesium mixed phosphates).