In this paper, the decomposition of H3PW12O40 in aqueous solution or in mixed solutions of water–ethanol or water–acetone is investigated by potentiometric titration and 31P NMR. Identification of the products from H3PW12O40 decomposition over a pH range of 1–12 was achieved using preparation high performance liquid chromatography (Pre-HPLC) combined with IR, UV–vis spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP). It is found that H3PW12O40 in aqueous solution decomposes in a stepwise fashion with increasing pH, with the following solution compositions: [PW12O40]3– (at pH ~ 1) [Formula: see text] [PW12O40]3– + [P2W21O71]6– + [PW11O39]7– (at pH 2.2) [Formula: see text] [PW12O40]3– + [P2W21O71]6– + [PW11O39]7– + [P2W18O62]6– + [P2W19O67]10– (at pH 3.5) [Formula: see text] [P2W21O71]6– + [PW11O39]7– + [P2W18O62]6– (at pH 5.4) [Formula: see text] [PW9O34]9– (at pH 7.3) [Formula: see text] PO43– + WO42– (pH > 8.3). In the first stages at pH < 8, H3PW12O40 decomposes partially with removal of W=O units. In the second stage at pH > 8, tungstophosphoric completely decomposes to PO43–. In contrast, the decomposition of H3PW12O40 is reduced, or the stability of the [PW12O40]3– anion is enhanced, in ethanol–water or acetone solution at pH < 8. Key words: 12-tungstophosphate heteropolyacid, decomposition behaviour, potentiometric titration, 31P NMR, preparation high performance liquid chromatography.