ZEOLITES derive their catalytic activity from the strong acidity of protons attached to the negatively charged aluminosilicate frame-work, which makes the materials excellent proton donors. Unlike zeolites, the aluminophosphate molecular sieves1,2 are built from alternating AIO4 and PO tetrahedra and are thus electrically neutral. Much attention has therefore been devoted to the genera-tion of Br0nsted acidity in these materials by introducing het-eroatoms, such as Si, Mg, Fe, Co or Zn, to produce negatively charged frameworks3-6. Similar arguments apply to gallophos-phate molecular sieves7-10, of which cioverite9,to is a remarkable example. This extra-large-pore material contains pore openings in the form of a four-leafed clover, defined by a ring of 20 gallium and phosphorus atoms, some of which are linked to terminal hydroxyl groups. Here we use NMR, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy to show that the P-OH groups in cloverite are localized versions of those in solid phosphoric acid, H3PO4. Cloverite is thus a strong Br0nsted acid even though no heteroatoms are present in its framework.