The treatment of A1PO(4)-5 with aqueous solutions of ammonium hexafluorosilicate, (NH4)(2)SiF6, at 75 degrees C for 3 h has been monitored using nuclear magnetic resonance, X-ray diffraction, and infrared spectroscopy. NMR shows that both Al and P are present in the filtrate. The treatment results not only in the isomorphous substitution of Si in the framework (lines at ca. -94 and -110 ppm) but also in the formation of defect sites. The frameworks of dealuminated samples are distorted in comparison with the parent A1PO(4)-5 as a result of strong interactions with the acetate cations. After calcination, the cell parameters are typical of A1PO(4)-5.