A series of aluminium orthophosphate-caesium oxide systems with various caesium oxide loadings (5-30 wt%) were prepared by impregnation of AlPO ₄ with a methanolic solution of caesium acetate and characterized by TG/DTA, XRD, DRIFT, Raman, SEM-EDX, XPS, ²⁷ Al and ³¹ P MAS NMR and nitrogen adsorption. Aluminium orthophosphate remained amorphous with caesium oxide incorporation when calcined at 873 K for 3 h. After thermal treatment at 1423 K it crystallized in the tridymite form except for materials containing 30 wt% caesium oxide. DRIFT spectroscopy showed that the P-OH stretching vibration at 3670 cm ^–1 decreased in intensity with caesium oxide loading. Materials at 20-30 wt% caesium oxide did not exhibit any hydroxyl bands. Besides, Al and P atoms remained in tetrahedral coordination as in unmodified aluminium orthophosphate. Moreover, the incorporation of caesium oxide leads, simultaneously, to a progressive decrease in surface area and pore volume (larger at 30 wt% caesium oxide) as well as to an increase in the most frequently occurring pore radius. On the other hand, caesium oxide reduced both the number and strength of acid sites as the caesium content increased. Consequently, 2-methylbut-3-yn-2-ol (MBOH) underwent almost exclusively dehydration to 3-methyl-3-buten-1-yne (acid activity) on pure AlPO ₄ , whereas its modification with increasing amounts of caesium oxide developed AlPO ₄ -based materials with increased basic properties and hence high selectivities (99 mol%) to the base-catalysed cleavage of MBOH yielding acetone and acetylene.