Acidity measurements by C-13 NMR spectroscopy with mesityl oxide (2) as indicator (the Delta delta method) have been conducted on phosphotungstic acid, H3PW12O40(1) at various levels of hydration, H3PW12O40.xH(2)O (1b) in concentrated solution in acetic acid. Extrapolation to infinite dilution of indicator allowed the determination of the H-0 acidity function. A comparison with the strong acids, sulfuric and perchloric, indicate that even in this solvent of low basicity the three hydrons of 1 dissociate independently, rather than consecutively as considered previously. The molecule of the heteropolyacid is thus equivalent to three molecules of strong acid in solution and behaves in essence like the solid acids, having acid sites of the same strength. Comparison with other acids has to be made at triple concentration of the latter and shows that the complex acid 1 is significantly stronger than perchloric acid, which in turn is stronger than sulfuric acid, as already known.