The precursor of the mixed metal oxide catalyst composed of H3PO4-WO3-Nb2O5, which exhibits excellent activity in Friedel-Crafts alkylations, was identified with P-31 NMR. It was revealed that the Keggin-type mixed heteropoly acid, H4PNbW11O40, was spontaneously generated during preparation of the H3PO4-WO3-Nb2O5 catalyst. The partial decomposition of H4PNbW11O40 occurred in the temperature range of 673823 K to give an amorphous oxide that had a Bronsted acid character. Based on the results of structural and acidic studies, the active species obtained through the calcination of H3PO4-WO3-Nb2O5 was ascribed to the transient state in the course of the thermal decomposition of the H4PNbW11O40 over the WO3-Nb2O5 support. In fact, the catalyst prepared by mixing H4PNbW11O40, niobic oxalate, and ammonium tungstate solutions, followed by calcination at 773 K, exhibited excellent activity in the Friedel-Crafts acylation of anisole with carboxylic acids. (c) 2006 Elsevier Inc. All rights reserved.